{"id":427,"date":"2007-03-27T11:46:36","date_gmt":"2007-03-27T16:46:36","guid":{"rendered":"\/?p=427"},"modified":"2013-01-23T10:49:12","modified_gmt":"2013-01-23T15:49:12","slug":"the-ipcc-sea-level-numbers","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2007\/03\/the-ipcc-sea-level-numbers\/","title":{"rendered":"The IPCC sea level numbers Os n\u00fameros do n\u00edvel do mar do IPCC<\/lang_po>"},"content":{"rendered":"
\n

The sea level rise numbers published in the new IPCC report<\/a> (the Fourth Assessment Report, AR4) have already caused considerable confusion. Many media articles and weblogs suggested there is good news on the sea level issue, with future sea level rise expected to be a lot less compared to the previous IPCC report<\/a> (the Third Assessment Report, TAR). Some articles reported that IPCC had reduced its sea level projection from 88 cm to 59 cm (35 inches to 23 inches) , some even said it was reduced from 88 cm to 43 cm (17 inches), and there were several other versions as well (see “Broad Irony”<\/a>). These statements are not correct and the new range up to 59 cm is not the full story. Here I will try to clarify what IPCC actually said and how these numbers were derived. (But if you want to skip the details, you can go straight to the critique<\/a> or the bottom line<\/a>).
\n<\/p>\n

What does IPCC say?<\/strong><\/p>\n

The Summary for Policy Makers<\/a> (SPM) released last month provides the following table of sea level rise projections:<\/p>\n\n\n\n\n\n\n\n\n\n
<\/td>\n\tSea Level Rise
\n(m at 2090-2099 relative to 1980-1999)<\/td>\n<\/tr>\n
Case<\/td>\nModel-based range
\nexcluding future rapid dynamical
\nchanges in ice flow <\/td>\n<\/tr>\n
B1 scenario \t<\/td>\n0.18 \u2013 0.38 <\/td>\n<\/tr>\n
A1T scenario \t<\/td>\n0.20 \u2013 0.45 <\/td>\n<\/tr>\n
B2 scenario \t<\/td>\n0.20 \u2013 0.43 <\/td>\n<\/tr>\n
A1B scenario \t<\/td>\n0.21 \u2013 0.48 <\/td>\n<\/tr>\n
A2 scenario \t<\/td>\n0.23 \u2013 0.51 <\/td>\n<\/tr>\n
A1FI scenario \t<\/td>\n0.26 \u2013 0.59 <\/td>\n<\/tr>\n<\/table>\n

It is this table on which the often-cited range of 18 to 59 cm is based. The accompanying text reads:<\/p>\n

\u2022 Model-based projections of global average sea level rise at the end of the 21st century (2090-2099) are shown in Table SPM-3. For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099. The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions15<\/sup>. {10.6}.<\/p>\n

Footnote 15: TAR projections were made for 2100, whereas projections in this Report are for 2090-2099. The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.<\/small><\/p>\n

\u2022 Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet flow, because a basis in published literature is lacking. The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM-3 would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise. {10.6}<\/p>\n

\u2022 If radiative forcing were to be stabilized in 2100 at A1B levels, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980\u20131999). Thermal expansion would continue for many centuries, due to the time required to transport heat into the deep ocean. {10.7}<\/p>\n

\u2022 Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. Current models suggest ice mass losses increase with temperature more rapidly than gains due to precipitation and that the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6\u00b0C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m. The corresponding future temperatures in Greenland are comparable to those inferred for the last interglacial period 125,000 years ago, when paleoclimatic information suggests reductions of polar land ice extent and 4 to 6 m of sea level rise. {6.4, 10.7}<\/p>\n

\u2022 Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude. {4.6, 10.7}<\/p>\n

\u2022 Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance. {10.7}<\/p>\n

\u2022 Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere. {7.3, 10.3}<\/p><\/blockquote>\n

(The above quotes document everything the SPM says about future sea level rise. The numbers in wavy brackets refer to the chapters of the full report, to be released in May.)<\/p>\n

What is included in these sea level numbers?<\/strong><\/p>\n

Let us have a look at how these numbers were derived. They are made up of four components: thermal expansion, glaciers and ice caps (those exclude the Greenland and Antarctic ice sheets), ice sheet surface mass balance, and ice sheet dynamical imbalance.<\/p>\n

1.\tThermal expansion (warmer ocean water takes up more space) is computed from coupled climate models. These include ocean circulation models and can thus estimate where and how fast the surface warming penetrates into the ocean depths.<\/p>\n

2.\tThe contribution from glaciers and ice caps (not including Greenland and Antarctica), on the other hand, is computed from a simple empirical formula linking global mean temperature to mass loss (equivalent to a rate of sea level rise), based on observed data from 1963 to 2003. This takes into account that glaciers slowly disappear and therefore stop contributing \u2013 the total amount of glacier ice left is actually only enough to raise sea level by 15-37 cm.<\/p>\n

3.\tThe contribution from the two major ice sheets is split into two parts. What is called surface mass balance refers simply to snowfall minus surface ablation (ablation is melting plus sublimation). This is computed from an ice sheet surface mass balance model, with the snowfall amounts and temperatures derived from a high-resolution atmospheric circulation model. This is not the same as the coupled models used for the IPCC temperature projections, so results from this model are scaled to mimic different coupled models and different climate scenarios. (A fine point: this surface mass balance does include some \u201cslow\u201d changes in ice flow, but this is a minor contribution.)<\/p>\n

4.\tFinally, there is another way how ice sheets can contribute to sea level rise: rather than melting at the surface, they can start to flow more rapidly. This is in fact increasingly observed around the edges of Greenland and Antarctica in recent years: outlet glaciers and ice streams that drain the ice sheets have greatly accelerated their flow. Numerous processes contribute to this, including the removal of buttressing ice shelves (i.e., ice tongues floating on water but in places anchored on islands or underwater rocks) or the lubrication of the ice sheet base by meltwater trickling down from the surface through cracks. These processes cannot yet be properly modelled<\/a>, but observations suggest that they have contributed 0 \u2013 0.7 mm\/year to sea level rise during the period 1993-2003. The projections in the table given above assume that this contribution simply remains constant until the end of this century.<\/p>\n

As an example, take the A1FI scenario \u2013 this is the warmest and therefore defines the upper limits of the sea level range. The \u201cbest\u201d estimates for this scenario are 28 cm for thermal expansion, 12 cm for glaciers and -3 cm for the ice sheet mass balance \u2013 note the IPCC still assumes that Antarctica gains more mass in this manner than Greenland loses. Added to this is a term according to (4) simply based on the assumption that the accelerated ice flow observed 1993-2003 remains constant ever after, adding another 3 cm by the year 2095. In total, this adds up to 40 cm, with an ice sheet contribution of zero. (Another fine point: This is slightly less than the central estimate of 43 cm for the A1FI scenario that was reported in the media, taken from earlier drafts of the SPM, because those 43 cm was not the sum of the individual best estimates for the different contributing factors, but rather it was the mid-point of the uncertainty range, which is slightly higher as some uncertainties are skewed towards high values.)<\/p>\n

How do the new numbers compare to the previous report?<\/strong><\/p>\n

<\/a>
\nSea level rise as observed (from Church and White 2006<\/a>) shown in red up to the year 2001, together with the IPCC (2001) scenarios for 1990-2100. See second figure below for a zoom into the period of overlap.<\/em><\/p>\n

The TAR showed sea level rise curves for a range of emission scenarios (shown in the Figure above together with the new observational record of Church and White 2006<\/a>). The range was based on simulations with a simple model (the MAGICC model) tuned to mimic the behaviour of a range of different complex climate models (e.g. in terms of different climate sensitivities ranging from 1.7 to 4.2 \u00baC), combined with simple equations for the glacier and ice sheet mass balances (\u201cdegree-days scheme\u201d). This model-based range is shown as the grey band (labelled \u201cSeveral models all SRES envelope\u201d in the original Figure 5<\/a> of the TAR SPM) and ranged from 21 to 70 cm, while the central estimate for each emission scenario is shown as a coloured dashed line. The largest central estimate of sea level rise is for the A1FI scenario (purple, 49 cm).
\nIn addition, the dashed grey lines indicate additional uncertainty in ice sheet behaviour. These lines were labelled \u201cAll SRES envelope including land ice uncertainty\u201d in the TAR SPM and extended the range up to 88 cm, adding 18 cm at the top end. One has to delve deeply into the appendix of Chapter 11 of the TAR to find out what these extra 18 cm entail: they include a \u201cmass balance uncertainty\u201d and an \u201cice dynamic uncertainty\u201d, where the latter is simply assumed to be 10% of the total computed mass loss of the Greenland ice sheet. Note that such an ice dynamic uncertainty was only included for Greenland but not for Antarctica; instability of the West Antarctic Ice Sheet, a scenario considered \u201cvery unlikely\u201d in the TAR, was explicitly not included in the upper limit of 88 cm.<\/p>\n

As we mentioned in our post on the release of the SPM<\/a>, it is apples and oranges to say that IPCC reduced the upper sea level limit from 88 cm to 59 cm, as the former included \u201cice dynamic uncertainty\u201d (albeit only for Greenland, as rapid ice flow changes in Antarctica were considered too unlikely to bother at the time), while the latter discusses this ice flow uncertainty separately in the text, stating it could add 10 cm, 20 cm or even more to the 59 cm in the table.<\/p>\n

So is it better to compare the model-based range 21 – 70 cm from the TAR to the 18 – 59 cm from the AR4? Even that is apples and oranges. For one, TAR cites the rise up to the year 2100, the AR4 up to the period 2090-2099, thus missing the last 5 years (or 5.5 years, but let\u2019s not get too pedantic) of sea level rise. For 2095, the TAR projection reduces from 70 cm to 65 cm (the central estimate for A1FI reduces from 49 cm to 46 cm). Also, the TAR range is a 95% confidence interval, the AR4 range a narrower 90% confidence interval. Giving the TAR numbers also as 90% ranges shaves another 3 cm off the top end. <\/p>\n

Sounds complicated? There are some more technical differences… but I will spare you those. The Paris IPCC meeting actually discussed the request from some delegates to provide a direct comparison of the AR4 and TAR numbers, but declined to do this in detail for being too complicated. The result was the two statements: <\/p>\n

The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.<\/p><\/blockquote>\n

and<\/p>\n

For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099.<\/p><\/blockquote>\n

(In fact delegates were told by the IPCC authors in Paris that with the new AR4 models, the central estimate for each scenario is slightly higher <\/em>that with the old models, if numbers are reported in a comparable manner.)<\/p>\n

The bottom line is thus that the methods have significantly improved (which is the reason behind all those methodological changes), but the expectation of how much sea level will rise in the coming century has not significantly changed. The biggest change is that ice sheet dynamics look more uncertain now than at the time of the TAR, which is why this uncertainty is not included any more in the cited range but discussed separately in the text.<\/p>\n

Critique – Could these numbers underestimate future sea level rise?<\/strong><\/a><\/p>\n

There\u2019s a number of issues worth discussing about these sea level numbers.<\/p>\n

The first is the treatment of potential rapid changes in ice flow (item 4 on the list above). The AR4 notes that the ice sheets have been losing mass recently<\/a> (the analysis period is 1993-2003). Greenland has contributed +0.14 to +0.28 mm\/year of sea level rise over this period, while for Antarctica the uncertainty range is -0.14 to +0.55 mm\/year. It is noted that the mass loss of Antarctica is mostly or entirely due to recent changes in ice flow. The question then is: how much will this process contribute to future sea level rise? The honest answer is: we don\u2019t know. As the SPM states, by the year 2095 it could be 10 cm. Or 20 cm. Or more. Or less. <\/p>\n

The IPCC included one <\/em>guess into the \u201cmodel-based range\u201d provided in the table: it took half of the Greenland mass loss and the whole Antarctic mass loss for 1993-2003, and assumed this would remain constant ever after until 2100. This assumption in my view has no scientific basis, as the ice-flow is almost certainly highly variable in time. The report itself states that this ice loss is due to a recent acceleration of flow, and that in 2005 it was already higher, and that in future the numbers could be several times higher \u2013 or they could be lower. Adding such an ill-founded number into the \u201cmodel-based\u201d range degrades the much more reliable estimates for thermal expansion, mountain glaciers and mass balance. Even worse: to numbers with error estimates, it adds a number without proper error estimate (the observational uncertainty for 1993-2003 is included, but who would claim this is an error estimation for future ice flow changes?). And then it presents only the combined error margins \u2013 you will notice that no central estimate is provided in the above table. If I had presented this as an error calculation in a first-semester physics assignment, I doubt I would have gotten away with it. The German delegation in Paris (of which I was a member) therefore suggested taking this ice-flow estimate out of the tabulated range. The numbers would have become slightly lower, but this approach would not have mixed up very different levels of uncertainty, and it would have been clear what is included in the table and what is not (namely ice flow changes), rather than attempting to partially include ice flow changes. The ice flow changes could have been discussed in the text \u2013 stating there that at the 1993-2003 rate, this term would contribute 3 cm by 2095, but it is bound to change and could turn out to be 10 cm or 20 cm or more. However, we found no support for this proposal, which would not have changed the science in any way but improved the clarity of presentation. <\/p>\n

As it is now, because of the complex and opaque way of combining the errors, even I could not tell you by how much the upper limit of 59 cm would be reduced if the questionable ice flow estimate was taken out, and one of the reasons provided by the IPCC authors for not adopting our proposal was that the numbers could not be calculated quickly.<\/p>\n

A second problem with the above range is that the models used to derive this projection significantly underestimate past sea level rise. We tried in vain to get this mentioned in the SPM, so you have to go to the main report to find this information. The AR4 states that for the period 1961-2003, the models on average give a rise of 1.2 mm\/year, while the data show 1.8 mm\/year, i.e. a 50% faster rise. This is despite using observed ice sheet mass loss (0.19 mm\/year) in the \u201cmodelled\u201d number in this comparison, otherwise the discrepancy would be even larger \u2013 the ice sheet models predict that the ice sheets gain mass due to global warming. The comparison looks somewhat better for the period 1993-2003, where the \u201cmodels\u201d give a rise of 2.6 mm\/year while the data give 3.1 mm\/year. But again the \u201cmodels\u201d estimate includes an observed ice sheet mass loss term of 0.41 mm\/year whereas ice sheet models give a mass gain of 0.1 mm\/year for this period; considering this, observed rise is again 50% faster than the best model estimate for this period. This underestimation carries over from the TAR models (see Rahmstorf et al. 2007<\/a> and the Figure below) \u2013 this is not surprising, since the new models give essentially the same results as the old models, as discussed above. <\/p>\n

<\/a>
\nComparison of the 2001 IPCC sea-level scenarios (starting in 1990) and observed data: the Church and White (2006) data based primarily on tide gauges (annual, red) and the satellite altimeter data (updated from Cazenave and Nerem 2004<\/a>, 3-month data spacing, blue, up to mid-2006) are shown with their trend lines. Note that the observed sea level rise tends to follow the uppermost dashed line of the IPCC scenarios, namely the one “including land ice uncertainty”, see first Figure.<\/em><\/p>\n

We therefore see that sea level appears to be rising about 50% faster than models suggest \u2013 consistently for the 1961-2003 and the 1993-2003 periods, and for the TAR models and the AR4 models. This could have a number of different reasons, and the discrepancy could be considered not significant given the error ranges of observations and models. It is no proof that models underestimate future sea level rise. But it is at least a plausible possibility that the models may underestimate future rise.<\/p>\n

A third issue worth mentioning is that of carbon cycle feedback. The temperature projections provided in table SPM-3 of the Summary for Policy Makers range from 1.1 to 6.4 \u00baC warming and include carbon cycle feedback. The sea level range, however, is based on scenarios that exclude this feedback and thus only range up to 4.5<\/s> 5.2 \u00baC. This could easily be misunderstood, as in table SPM-3 the temperature ranges including carbon cycle feedback are shown right next to the sea level ranges, but the latter actually apply to a smaller temperature range. As a rough estimate, I suggest that for a 6.4 \u00baC warming scenario, of the order of 20<\/s> 15 cm would have to be added to the 59 cm defining the upper end of the sea level range.<\/p>\n

A final point is the regional aspects. Planners of coastal defences need to be aware that sea level rise will not be the same everywhere. The AR4 shows a map of regional sea level changes, which shows that e.g. European coasts can expect a rise by 5-15 cm more than the global mean rise \u2013 that is a model average, not including an uncertainty range. The pattern in this map is remarkably similar to that expected from a slowdown in thermohaline circulation (see Levermann et al. 2005<\/a>) so probably it is dominated by this effect. In addition, some land areas are rising and some are subsiding in response to the end of the last Ice Age or due to local anthropogenic processes (e.g. groundwater withdrawal), which local planners need to account for.<\/p>\n

The bottom line<\/strong><\/a><\/p>\n

The main conclusion of this analysis is that sea level uncertainty is not smaller now than it was at the time of the TAR, and that quoting the 18-59 cm range of sea level rise, as many media articles have done, is not telling the full story. 59 cm is unfortunately not the \u201cworst case\u201d. It does not include the full ice sheet uncertainty, which could add 20 cm or even more. It does not cover the full \u201clikely\u201d temperature range given in the AR4 (up to 6.4 \u00baC) \u2013 correcting for that could again roughly add 20<\/s> 15 cm. It does not account for the fact that past sea level rise is underestimated by the models for reasons that are unclear. Considering these issues, a sea level rise exceeding one metre can in my view by no means ruled out. In a completely different analysis<\/a>, based only on a simple correlation of observed sea level rise and temperature, I came to a similar conclusion. As stated in that paper, my point here is not <\/em>that I predict that sea level rise will be higher than IPCC suggests, or that the IPCC estimates for sea level are wrong in any way. My point is that in terms of a risk assessment, the uncertainty range that one needs to consider is in my view substantially larger than 18-59 cm.<\/p>\n

A final thought: this discussion has all been about sea level rise until the year 2095. Sea level rise does not end there, as the quotes from the SPM at the beginning of this article show. Over several centuries, without serious mitigation efforts we may expect several meters of sea level rise. The Advisory Council on Global Change<\/a> of the German government (disclosure: I’m a member of this body) in its recent special report on the oceans<\/a> has proposed to limit long-term sea level rise to a maximum of one meter, as a guard-rail to guide climate policy. But that\u2019s another story.<\/p>\n

Update:<\/strong> I was just informed by one of the IPCC authors that the temperature scenarios without carbon cycle feedback range up to 5.2 \u00baC, not 4.5 \u00baC as I had assumed. This number is not found in the IPCC report; I had tried to interpret it from a graph, but not accurately enough. My apologies! The numbers in the text above that had to be corrected are marked by strikethrough font. -stefan<\/p>\n

<\/p>\n

O aumento do n\u00edvel do mar publicado no novo relat\u00f3rio do IPCC<\/a> (o Quarto Relat\u00f3rio de Avalia\u00e7\u00e3o, AR4) j\u00e1 tem causado confus\u00e3o consider\u00e1vel. Muitos artigos da m\u00eddia sugerem que h\u00e1 boas not\u00edcias sobre a quest\u00e3o do n\u00edvel do mar, com previs\u00f5es muito menores de aumento do n\u00edvel do mar comparadas \u00e0s previs\u00f5es do relat\u00f3rio anterior do IPCC<\/a> (o Terceiro Relat\u00f3rio de Avalia\u00e7\u00e3o, TAR). Alguns artigos reportam que o IPCC reduziu a proje\u00e7\u00e3o para o aumento do n\u00edvel do mar de 88 para 59 cm, enquanto outros dizem que tal proje\u00e7\u00e3o teria sido reduzida de 88 para 43 cm, e existem muitas outras vers\u00f5es tamb\u00e9m (veja “Ampla Ironia”<\/a>). Tais declara\u00e7\u00f5es s\u00e3o incorretas dado que o novo valor de at\u00e9 59 cm n\u00e3o representa sequer toda a est\u00f3ria. Aqui tentarei clarear o que o IPCC de fato quer dizer e como esses n\u00fameros s\u00e3o derivados. (Mas caso prefira pular os detalhes, v\u00e1 direto para a cr\u00edtica<\/a> ou a \u00faltima linha<\/a>).
<\/a><\/p>\n

O que o IPCC diz?<\/strong><\/p>\n

O Sum\u00e1rio para Tomadores de Decis\u00e3o<\/a> (SPM) lan\u00e7ado no ultimo m\u00eas fornece a seguinte tabela de proje\u00e7\u00f5es para o aumento do n\u00edvel do mar:<\/p>\n\n\n\n\n\n\n\n\n\n\n
<\/td>\n\tAumento do N\u00edvel do Mar
\n(em metros para 2090-2099
relativo a 1980-1999)<\/td>\n<\/tr>\n
Caso<\/td>\nIntervalo baseado em modelo
excetuando-se r\u00e1pidas mudan\u00e7as
futuras no fluxo de gelo<\/td>\n<\/tr>\n
Cen\u00e1rio B1 \t<\/td>\n0.18 \u2013 0.38 <\/td>\n<\/tr>\n
Cen\u00e1rio A1T \t<\/td>\n0.20 \u2013 0.45 <\/td>\n<\/tr>\n
Cen\u00e1rio B2 \t<\/td>\n0.20 \u2013 0.43 <\/td>\n<\/tr>\n
Cen\u00e1rio A1B \t<\/td>\n0.21 \u2013 0.48 <\/td>\n<\/tr>\n
Cen\u00e1rio A2 \t<\/td>\n0.23 \u2013 0.51 <\/td>\n<\/tr>\n
Cen\u00e1rio A1FI \t<\/td>\n0.26 \u2013 0.59 <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\u00c9 desta tabela que sai o usualmente citado intervalo de 18 a 59 cm. O texto que acompanha a tabela diz:<\/p>\n

\n

\u2022
\nProje\u00e7\u00f5es baseadas em modelos da eleva\u00e7\u00e3o do n\u00edvel do mar no final do s\u00e9culo XXI (2090-2099) s\u00e3o mostradas na Tabela SPM-3. Para cada cen\u00e1rio, o ponto m\u00e9dio do intervalo na Tabela SPM-3 situa-se dentro de 10% da m\u00e9dia do modelo do TAR para 2090-2099. Os intervalos s\u00e3o mais estreitos que no TAR principalmente devido \u00e0s melhorias na informa\u00e7\u00e3o sobre algumas incertezas nas contribui\u00e7\u00f5es projetadas15<\/sup>. {10.6}.<\/p>\n

nota de rodap\u00e9
\n15: As poje\u00e7\u00f5es no TAR foram feitas para 2100, enquanto que as proje\u00e7\u00f5es desse relat\u00f3rio s\u00e3o para 2090-2099. O TAR deveria apresentar intervalos similares aos da Tabela SPM-3 se as incertezas tivessem sido tratadas da mesma maneira.<\/small><\/p>\n

\u2022 Os modelos atuais n\u00e3o incluem incertezas do feedback clim\u00e1tico do ciclo do carbono e t\u00e3o pouco incluem efeitos completos das mudan\u00e7as dos fluxos das placas de gelo, dado que ainda faltam fundamentos publicados na literatura. As proje\u00e7\u00f5es incluem uma contribui\u00e7\u00e3o devido ao aumento do fluxo de gelo da Groenl\u00e2ndia e Ant\u00e1rtica em taxas observadas para 1993-2003, mas tais taxas de fluxo poderiam aumentar ou diminuir no futuro. Por exemplo, se essa contribui\u00e7\u00e3o crescer linearmente com a mudan\u00e7a da temperatura m\u00e9dia global, os intervalos superiores da eleva\u00e7\u00e3o do n\u00edvel do mar nos cen\u00e1rios SRES (Relat\u00f3rio Especial dos Cen\u00e1rios de Emiss\u00e3o do IPCC) mostrados na Tabela SPM-3 deveriam aumentar em 0.1 m a 0.2 m. Valores maiores n\u00e3o podem ser exclu\u00eddos, mas o conhecimento desses efeitos \u00e9 muito limitado para avaliar suas probabilidades ou fornecer uma melhor estimativa ou um limite superior para o aumento do n\u00edvel do mar. {10.6}<\/p>\n

\u2022 Se a for\u00e7ante radiativa fosse estabilizar em 2100 em n\u00edveis estimados no cen\u00e1rio A1B, a expans\u00e3o t\u00e9rmica somente levaria a um aumento do n\u00edvel do mar de 0.3 a 0.8 m em 2300 (relativo a 1980\u20131999). A expans\u00e3o t\u00e9rmica continuaria por muitos s\u00e9culos, devido ao tempo requerido para transportar calor para o oceano profundo. {10.7}<\/p>\n

\u2022 A contra\u00e7\u00e3o da camada de gelo da Groenl\u00e2ndia \u00e9 projetada a continuar contribuindo para o aumento do n\u00edvel o mar ap\u00f3s 2100. Os modelos atuais sugerem que um aumento da perda de massa de gelo com a temperatura seria mais r\u00e1pido do que um ganho de massa de gelo com a precipita\u00e7\u00e3o, e que o balan\u00e7o de massa da superf\u00edcie tornaria-se negativo sob um aquecimento global m\u00e9dio (relativo aos valores pr\u00e9-industriais) excedendo 1.9 a 4.6\u00b0C. Se um balan\u00e7o negativo de massa da superf\u00edcie fosse sutentado por mil\u00eanios, isso levaria a uma elimina\u00e7\u00e3o virtualmente completa da cobertura de gelo da Groenl\u00e2ndia e uma contribui\u00e7\u00e3o resultante do aumento do n\u00edvel do mar ao redor de 7 m. As temperaturas futuras correspondentes na Groenl\u00e2ndia s\u00e3o compar\u00e1veis \u00e0quelas inferidas para o \u00faltimo per\u00edodo interglacial h\u00e1 125 mil anos atr\u00e1s, quando as informa\u00e7\u00f5es paleoclim\u00e1ticas sugerem uma redu\u00e7\u00e3o da extens\u00e3o de gelo polar e um aumento do n\u00edvel do mar de 4 a 6 m. {6.4, 10.7}<\/p>\n

\u2022 Processos din\u00e2micos relacionados o fluxo de gelo n\u00e3o inclu\u00eddos nos modelos atuais mas sugeridos por recentes observa\u00e7\u00f5es poderia aumentar a vulnerabilidade das placas de gelo ao aquecimento, aumentando a eleva\u00e7\u00e3o do n\u00edvel do mar no futuro. A compreens\u00e3o desses processos \u00e9 limitada e n\u00e3o h\u00e1 consenso sobre sua magnitude. {4.6, 10.7}<\/p>\n

\u2022 Estudos atuais de modelos globais projetam que a camada de gelo Ant\u00e1rtica pode permanecer muito fria para um amplo derretimento superficial e espera-se um ganho de massa devido a um aumento de queda de neve. Contudo, uma perda l\u00edquida de gelo poderia ocorrer se uma descarga din\u00e2mica de gelo dominar o balan\u00e7o de massa da camada de gelo. {10.7}<\/p>\n

\u2022 Ambas as emiss\u00f5es antropog\u00eanicas passadas e futuras de di\u00f3xido de carbono dever\u00e3o continuar a contribuir no aquecimento e na eleva\u00e7\u00e3o do n\u00edvel do mar por mais de um mil\u00eanio, por conta da escala de tempo requerida para a remo\u00e7\u00e3o desse g\u00e1s da atmosfera. {7.3, 10.3}<\/p>\n<\/blockquote>\n

(Os itens acima documentam tudo que o SPM diz sobre o futuro da eleva\u00e7\u00e3o do n\u00edvel do mar. Os n\u00fameros entre chaves refem-se aos cap\u00edtulos do relat\u00f3rio completo a ser divulgado em maio.)<\/p>\n

O que est\u00e1 incluso nesses n\u00fameros de n\u00edvel do mar?<\/strong><\/p>\n

Vamos olhar como esses n\u00fameros s\u00e3o derivados. Eles s\u00e3o constitu\u00eddos de quatro componentes: expans\u00e3o t\u00e9rmica, geleiras e camadas de gelo (excetuando-se as capas de gelo da Groenl\u00e2ndia e Ant\u00e1rtica), balan\u00e7o de massa de placas de gelo superficiais, e o desbalan\u00e7o din\u00e2mico das placas de gelo.<\/p>\n

1. Expans\u00e3o t\u00e9rmica (\u00e1gua oce\u00e2nica mais quente ocupa maior espa\u00e7o) \u00e9 computada de modelos clim\u00e1ticos acoplados. Esses incluem modelos de circula\u00e7\u00e3o oce\u00e2nica e podem assim estimar onde e qu\u00e3o r\u00e1pido o aquecimento superficial penetra nos oceanos profundos.<\/p>\n

2. A contribui\u00e7\u00e3o de geleiras e camadas de gelo (n\u00e3o incluindo Groenl\u00e2ndia e Ant\u00e1rtica), por sua vez, \u00e9 computada de uma simples formula\u00e7\u00e3o emp\u00edrica que liga a temperatura m\u00e9dia global \u00e0 perda de massa (equivalente a uma taxa de eleva\u00e7\u00e3o do n\u00edvel do mar), baseada em dados observados entre 1963 e 2003. Tal formula\u00e7\u00e3o considera que as geleiras desaparecem vagarosamente e conseq\u00fcentemente param de contribuir \u2013 a quantidade total de geleiras remanecente seria suficiente para elevar o n\u00edvel do mar em 15-37 cm.<\/p>\n

3. A contribui\u00e7\u00e3o das duas maiores coberturas de gelo \u00e9 dividida em duas partes. O que \u00e9 chamado de balan\u00e7o de massa superficial se refere simplesmente a queda de neve menos a abla\u00e7\u00e3o de gelo superficial (que \u00e9 o derretimento somado \u00e0 sublima\u00e7\u00e3o). Este \u00e9 computado por um modelo de balan\u00e7o de massa de placa de gelo superficial, com as quantidades de queda de neve e temperaturas derivados de um modelo de alta resolu\u00e7\u00e3o da circula\u00e7\u00e3o atmosf\u00e9rica. Este c\u00e1lculo n\u00e3o \u00e9 o mesmo dos modelos acoplados usados nas proje\u00e7\u00f5es de temperatura do IPCC, de modo que os resultados desse modelo s\u00e3o ajustados para mimetizar diferentes modelos acoplados e diferentes cen\u00e1rios clim\u00e1ticos. (Um importante detalhe: esse balan\u00e7o de massa superficial inclui algumas mudan\u00e7as \u201cmorosas\u201d no fluxo de gelo, mas essa \u00e9 uma pequena contribui\u00e7\u00e3o.)<\/p>\n

4. Finalmente, existe um outro modo pelo qual as placas de gelo podem contribuir para a eleva\u00e7\u00e3o do n\u00edvel do mar: ao inv\u00e9s de derreterem na superf\u00edcie, podem come\u00e7ar a fluir mais rapidamente. Isso vem sendo observado com freq\u00fc\u00eancia ao redor das bordas da Groenl\u00e2ndia e Ant\u00e1rtica em anos recentes: sa\u00eddas de geleiras e rios de gelo que drenam as placas de gelo t\u00eam aumentado suas vaz\u00f5es. Numerosos processos contribuem para isso, incluindo a remo\u00e7\u00e3o de conchas de gelo (i.e., gelos que flutuam sobre a \u00e1gua ancoradas em ilhas ou rochas submersas) ou a eros\u00e3o da base da placa de gelo por \u00e1gua l\u00edquida fluindo pela superf\u00edcie atrav\u00e9s de falhas no gelo. Tais processos n\u00e3o podem ainda ser adequadamente modelados<\/a>, mas as observa\u00e7\u00f5es sugerem que eles t\u00eam contribu\u00eddo com 0 \u2013 0.7 mm\/ano para a eleva\u00e7\u00e3o do n\u00edvel do mar no per\u00edodo 1993-2003. As proje\u00e7\u00f5es na dada tabela assumem que tal contribui\u00e7\u00e3o simplesmente se mant\u00e9m constante at\u00e9 o fim deste s\u00e9culo.<\/p>\n

Por exemplo, tome o cen\u00e1rio A1FI \u2013 este \u00e9 o mais quente e por isso define os limites superiores do intervalo do n\u00edvel do mar. A \u201cmelhor\u201d estimativa desse cen\u00e1rio \u00e9 28 cm para a expans\u00e3o t\u00e9rmica, 12 cm para as geleiras e -3 cm para o balan\u00e7o de massa das placas de gelo \u2013 note que o IPCC ainda assume que a Ant\u00e1rtica ganha mais massa atrav\u00e9s desse modo do que a Groenl\u00e2ndia perde. Adicionado a isso h\u00e1 um termo de acordo com (4) simplesmente baseado na premissa de que o acelerado fluxo de gelo observado em 1993-2003 se mant\u00e9m sempre constante, adicionando outros 3 cm em 2095. No total, isso totaliza at\u00e9 40 cm, com uma contribui\u00e7\u00e3o nula das placas de gelo. (Outro ponto importante: Isso representa um pouco menos do que a estimativa central de 43 cm para o cen\u00e1rio A1FI que foi divulgado na m\u00eddia, tirado dos primeiros rascunhos do SPM, pois estes 43 cm n\u00e3o eram a soma das melhores estimativas individuais para os diferentes fatores contribuintes, mas, ao contr\u00e1rio, era um ponto m\u00e9dio do intervalo das incertezas, o qual \u00e9 um pouco maior quando algumas incertezas s\u00e3o tomadas com valores mais altos.)<\/p>\n

Como esses n\u00fameros se comparam com o relat\u00f3rio anterior?<\/strong><\/p>\n

<\/a>
\nEleva\u00e7\u00e3o do n\u00edvel do mar como verificado em Church e White 2006<\/a> mostrado em vermelho at\u00e9 o ano de 2001, junto com os cen\u00e1rios do IPCC (2001) para 1990-2100. Veja a segunda figura abaixo para um zoom no per\u00edodo de sobreposi\u00e7\u00e3o.<\/em><\/p>\n

O TAR mostrou curvas de eleva\u00e7\u00e3o de n\u00edvel do mar para uma gama de cen\u00e1rios de emiss\u00e3o (mostrada na Figura acima junto com novos dados obervacionais de Church e White 2006<\/a>). Essa gama foi baseada em simula\u00e7\u00f5es com um modelo simples (o modelo MAGICC) ajustado para mimetizar o comportamento de uma gama de diferentes modelos clim\u00e1ticos complexos (por exemplo em termos de diferentes sensibilidades clim\u00e1ticas variando de 1.7 a 4.2 \u00baC), combinado com equa\u00e7\u00f5es simples para um glacial e balan\u00e7os de massa de placa de gelo (\u201cesquema graus-dias\u201d). Este intervalo baseado em modelo \u00e9 mostrado como uma banda verde (legendada como \u201cSeveral models all SRES envelope\u201d na Figura 5<\/a> original do TAR SPM) e variou de 21 a 70 cm, enquanto que a estimativa central para cada cen\u00e1rio de emiss\u00e3o \u00e9 mostrada como uma linha tracejada colorida. A maior estimativa central da eleva\u00e7\u00e3o do n\u00edvel do mar foi para o cen\u00e1rio A1FI (cor p\u00farpura, 49 cm).
\nAinda mais, as curvas tracejadas em cinza indicam incertezas adicionais no comportamento das placas de gelo. Tais linhas foram legendadas como \u201cAll SRES envelope including land ice uncertainty\u201d no TAR SPM e ampliou o intervalo at\u00e9 88 cm, adicionando 18 cm no limite superior. \u00c9 preciso procurar minuciosamente no ap\u00eandice do Cap\u00edtulo 11 do TAR para encontrar o que esses 18 cm extras representam: eles incluem uma \u201cincerteza no balan\u00e7o de massa\u201d e uma \u201cincerteza de din\u00e2mica de gelo\u201d, onde o \u00faltimo \u00e9 meramente assumido como 10% da perda de massa total computada para a placa de gelo da Groenl\u00e2ndia. Note que tal incerteza na din\u00e2mica de gelo foi somente inclu\u00edda para a Groenl\u00e2ndia mas n\u00e3o para a Ant\u00e1rtica; instabilidade da Placa de Gelo Oeste da Ant\u00e1rtica, um cen\u00e1rio considerado \u201cmuito improv\u00e1vel\u201d no TAR, foi explicitamente n\u00e3o inclu\u00eddo no limite superior de 88 cm.<\/p>\n

Como mencionamos em nossa postagem sobre a divulga\u00e7\u00e3o do SPM<\/a>, seria comparar ma\u00e7\u00e3s e laranjas ao dizer que o IPCC reduziu o limite superior do n\u00edvel do mar de 88 cm para 59 cm, a medida em que o primeiro incluiu \u201ca incerteza da din\u00e2mica do gelo\u201d (muito embora somente para a Groenl\u00e2ndia, pois mudan\u00e7as r\u00e1pidas do fluxo de gelo na Ant\u00e1rtica foram consideradas muito improv\u00e1veis para preocupar naquele tempo), enquanto que o segundo discute essa incerteza do fluxo de gelo separadamente no texo, declarando que isso poderia adicionar 10 cm, 20 cm ou ainda mais aos 59 cm da tabela.<\/p>\n

Assim seria melhor comparar o intervalo baseado em modelo de 21 – 70 cm do TAR com o 18 – 59 cm do AR4? Mesmo isso seria comparar ma\u00e7\u00e3s com laranjas. Para um, o TAR cita a eleva\u00e7\u00e3o at\u00e9 o ano 2100, o AR4 at\u00e9 o per\u00edodo 2090-2099, assim faltam os \u00faltimos cinco anos (ou 5.5 anos, mas n\u00e3o sejamos pedantes) da eleva\u00e7\u00e3o do n\u00edvel do mar. Para 2095, a proje\u00e7\u00e3o do TAR reduz de 70 cm para 65 cm (a estimativa central para o cen\u00e1rio A1FI reduz de 49 cm para 46 cm). Tamb\u00e9m, o intervalo do TAR \u00e9 um intervalo de 95% de confian\u00e7a, j\u00e1 o intervalo AR4 \u00e9 mais estreito para um intervalo de confian\u00e7a de 90%. Dados os n\u00fameros do TAR tamb\u00e9m como intervalos de 90% remove outros 3 cm do limite superior final. <\/p>\n

Parece complicado? Existem outras diferen\u00e7as mais t\u00e9cnicas… mas irei poupar-lhes disso. A reuni\u00e3o de Paris do IPCC j\u00e1 discutiu o pedido de alguns delegados de fornecer uma compara\u00e7\u00e3o direta dos n\u00fameros do AR4 e do TAR, mas desistiram de fazer isso detalhadamente por ser muito complicado. O resultado foi duas declara\u00e7\u00f5es:<\/p>\n

\n

O TAR deveria ter intervalos similares aos da Tabela SPM-3 se ele tivesse tratado as incertezas da mesma maneira. <\/p>\n<\/blockquote>\n

e<\/p>\n

\n

Para cada cen\u00e1rio, o ponto m\u00e9dio do intervalo na Tabela SPM-3 est\u00e1 dentro de 10% da m\u00e9dia do modelo TAR para 2090-2099.<\/p>\n<\/blockquote>\n

(Na verdade, foi dito aos delegados pelos autores do IPCC em Paris que com os novos modelos AR4, as estimativas centrais de cada cen\u00e1rio seriam um pouco maiores<\/em> que aquelas dos velhos modelos, se os n\u00fameros s\u00e3o reportados de forma compar\u00e1vel.)<\/p>\n

A \u00faltima linha mostra ent\u00e3o que os m\u00e9todos t\u00eam sido significativamente melhorados (raz\u00e3o por detr\u00e1s de todos essas mudan\u00e7as metodol\u00f3gicas), mas a expectativa de quanto o n\u00edvel do mar ir\u00e1 subir no s\u00e9culo que vir\u00e1 n\u00e3o mudou muito. A maior mudan\u00e7a \u00e9 que a din\u00e2mica das placas de gelo parecem mais incertas agora que no tempo do TAR, que \u00e9 a raz\u00e3o para que esta incerteza n\u00e3o seja mais inclusa nos intervalos citados, mas sim discutida separadamente no texto.<\/p>\n

Cr\u00edtica – Poderiam esses n\u00fameros subestimar a futura eleva\u00e7\u00e3o do n\u00edvel do mar?<\/strong><\/a><\/p>\n

Existem v\u00e1rias discuss\u00f5es importantes sobre os n\u00fameros do n\u00edvel do mar.<\/p>\n

O primeiro \u00e9 o tratamento das mudan\u00e7as r\u00e1pidas potenciais no fluxo de gelo (item 4 da lista acima). O AR4 aponta que as placas de gelo t\u00eam recentemente perdido massa<\/a> (o per\u00edodo de an\u00e1lise \u00e9 1993-2003). A Groenl\u00e2ndia tem contribu\u00eddo com +0.14 a +0.28 mm\/ano para a eleva\u00e7\u00e3o do n\u00edvel do mar sobre esse per\u00edodo, enquanto que para a Ant\u00e1rtica a incerteza varia de -0.14 a +0.55 mm\/ano. \u00c9 observado que a perda de massa da Ant\u00e1rtica \u00e9 predominante ou inteiramente devido \u00e0s recentes mudan\u00e7as do fluxo de gelo. A quest\u00e3o ent\u00e3o \u00e9: Quanto esse processo ir\u00e1 contribuir para o futuro da eleva\u00e7\u00e3o do n\u00edvel do mar? A resposta honesta \u00e9: n\u00f3s n\u00e3o sabemos. Como o SPM declara, pelo ano 2095 poderia ser 10 cm. Ou 20 cm. Ou mais. Ou menos. <\/p>\n

O IPCC incluiu uma <\/em> suposi\u00e7\u00e3o no ‘intervalo baseado em modelo’ dado na tabela: tal suposi\u00e7\u00e3o toma metade da perda de massa da Groenl\u00e2ndia e toda a perda de massa Ant\u00e1rtica para 1993-2003, e assume que as perdas se manteriam sempre constantes at\u00e9 2100. Essa permissa na minha vis\u00e3o n\u00e3o tem embasamento cient\u00edfico, pois o fluxo de gelo \u00e9 quase que certamente muito vari\u00e1vel no tempo. O relat\u00f3rio por si s\u00f3 declara que tal perda de gelo seja devida a uma acelera\u00e7\u00e3o recente do fluxo, e que em 2005 j\u00e1 era bastante alta, e no futuro os n\u00fameros poderiam ser v\u00e1rias vezes maior \u2013 ou poderiam ser menores. Incluindo um n\u00famero fundamentalmente deficiente no intervalo ‘baseado em modelo’ degrada estimativas muito mais confi\u00e1veis para a expans\u00e3o t\u00e9rmica, geleiras de montanhas e balan\u00e7o de massa. Ainda pior: para os n\u00fameros com estimativas de erro, \u00e9 adicionado um n\u00famero sem uma estimativa apropriada de erro (a incerteza observada para 1993-2003 \u00e9 inclu\u00edda, mas quem asseguraria que esta seja v\u00e1lida para futuras mudan\u00e7as no fluxo de gelo?). E ent\u00e3o s\u00e3o apresentadas somente as margens de erro combinadas \u2013 voc\u00ea pode notar que nenhuma estimativa central \u00e9 fornecida na tabela acima. Se eu tivesse apresentado isso como um erro de c\u00e1lculo numa li\u00e7\u00e3o de casa no primeiro semestre de f\u00edsica, duvido que eu conseguiria escapar disso. A delega\u00e7\u00e3o alem\u00e3 em Paris (da qual sou membro) ent\u00e3o sugeriu tirar a estimativa do fluxo de gelo do intervalo tabulado. Os n\u00fameros se tornariam um pouco menores, mas esta abordagem n\u00e3o mesclaria n\u00edveis muito diferentes de incerteza, e ficaria claro o que estaria incluso na tabela e o que n\u00e3o estaria (as mudan\u00e7as de fluxo de gelo), ao inv\u00e9s de tentar incluir parcialmente mudan\u00e7as nos fluxos de gelo. Tais mudan\u00e7as teriam sido discutidas no texto \u2013 dizendo que nas taxas de 1993-2003, tal termo poderia contribuir com 3 cm em 2095, mas esse valor poderia mudar para 10 cm ou 20 cm ou mais. Todavia, n\u00e3o encotramos nenhum suporte para esta proposta, a qual n\u00e3o teria mudado a Ci\u00e8ncia de maneira alguma, mas melhorado a claridade da apresenta\u00e7\u00e3o. <\/p>\n

Como est\u00e1 agora, devido \u00e0 forma complexa e obscura da combina\u00e7\u00e3o dos erros, at\u00e9 mesmo eu n\u00e3o poderia dizer por quanto o limite superior de 59 cm seria reduzido se a question\u00e1vel estimativa fosse removida, e uma das raz\u00f5es para que os autores do IPCC n\u00e3o adotassem nossa proposta foi a de que os n\u00fameros n\u00e3o poderiam ser calculados rapidamente.<\/p>\n

Um segundo problema com o intervalo acima \u00e9 que os modelos usados para derivar as proje\u00e7\u00f5es subestimam significativamente a eleva\u00e7\u00e3o do n\u00edvel do mar em tempos pret\u00e9ritos. Tentamos em v\u00e3o fazer isso ser mencionado no SPM, de modo que voc\u00ea teria que ir ao relat\u00f3rio principal para encontrar essa informa\u00e7\u00e3o. O AR4 declara que para o per\u00edodo 1961-2003, os modelos sobre as m\u00e9dias fornece uma eleva\u00e7\u00e3o de 1.2 mm\/ano, enquanto que os dados mostram 1.8 mm\/ano, i.e. um crescimento 50% mais r\u00e1pido. E isto sem considerar a taxa de perda de placa de gelo (0.19 mm\/ano) nos n\u00fameros ‘modelados’ nesta compara\u00e7\u00e3o. Se assim fosse, a discrep\u00e2ncia seria ainda maior \u2013 os modelos de placa de gelo prev\u00e8m que as placas de gelo ganhariam massa em fun\u00e7\u00e3o do aquecimento global. A compara\u00e7\u00e3o parece um pouco melhor no per\u00edodo de 1993-2003, para o qual os modelos fornecem uma eleva\u00e7\u00e3o de 2.6 mm\/ano enquanto os dados fornecem 3.1 mm\/ano. Mas de novo as estimativas de ‘modelos’ incluem uma observada perda de massa de gelo de 0.41 mm\/ano enquanto os modelos de placas de gelo fornecem um ganho de massa de 0.1 mm\/ano para esse per\u00edodo; considerando isso, a eleva\u00e7\u00e3o observada \u00e9 de novo 50% mais r\u00e1pida do que as melhores estimativas de modelos para esse per\u00edodo. Esta subestimativa persiste dos modelos do TAR (veja Rahmstorf et al. 2007<\/a> e Figura abaixo) \u2013 isso n\u00e3o \u00e9 uma surpresa, desde que os novos modelos d\u00e3o essencialmente os mesmos resultados dos modelos antigos, como discutido acima. <\/p>\n

<\/a>
\nCompara\u00e7\u00e3o dos cen\u00e1rios do n\u00edvel do mar do IPCC 2001 (com in\u00edcio em 1990) e dados observados: os dados de Church e White (2006) baseiam-se primariamente em esta\u00e7\u00f5es de medi\u00e7\u00e3o de mar\u00e9 (anual em vermelho) e dados de sat\u00e9lite alt\u00edmetro (atualizado de Cazenave e Nerem 2004<\/a>, dados espa\u00e7ados de 3 meses, em azul, at\u00e9 meados de 2006) s\u00e3o mostrados com suas linhas de tend\u00eancia. Note que a tend\u00eancia de eleva\u00e7\u00e3o do n\u00edvel do mar segue a linha tracejada mais superior dos cen\u00e1rios do IPCC, exatamente aquela nomeada “incluindo a incerteza de gelo terrestre”, veja a primeira figura. <\/em><\/p>\n

N\u00f3s ent\u00e3o vemos que o n\u00edvel do mar parece estar subindo cerca de 50% mais r\u00e1pido que os modelos sugerem – consistentemente para os per\u00edodos de 1961-2003 e 1993-2003, e para os modelos TAR e AR4. Isso pode ter diversas raz\u00f5es, e a discrep\u00e2ncia poderia ser considerada insignificante dados os intervalos de erros das oberva\u00e7\u00f5es e modelos. N\u00e3o h\u00e1 provas de que os modelos subestimam a eleva\u00e7\u00e3o o n\u00edvel do mar. Mas h\u00e1 no m\u00ednimo uma possibilidade plaus\u00edvel de que os modelos possam subestimar a eleva\u00e7\u00e3o futura.<\/p>\n

Uma terceira quest\u00e3o de import\u00e2ncia diz respeito ao feedback do ciclo do carbono. As proje\u00e7\u00f5es de temperatura fornecidas na tabela SPM-3 do Sum\u00e1rio para Tomadores de Decis\u00e3o variam de 1.1 a 6.4 \u00baC de aquecimento e inclui o feedback do ciclo do carbono. A varia\u00e7\u00e3o do n\u00edvel do mar, contudo, \u00e9 baseada em cen\u00e1rios que excluem esse feedback e assim variam somente at\u00e9 4.5<\/s> 5.2 \u00baC. Isso poderia facilmente ser mal interpretado, pois na tabela SPM-3 os intervalos de temperatura que incluem o feedback do ciclo do carbono s\u00e3o mostrados ao lado dos intervalos do n\u00edvel do mar, mas esses \u00faltimos na verdade aplicam-se a um menor intervalo de temperatura. Como uma estimativa grosseira, sugiro que para um cen\u00e1rio de aquecimento de 6.4 \u00baC, da ordem de 20<\/s> 15 cm deveria ser adicionado aos 59 cm para definir o limite superior do intervalo de eleva\u00e7\u00e3o do n\u00edvel do mar.<\/p>\n

Um ponto final seria os aspectos regionais. Gerentes de planejamento de zonas costeiras precisam ter conci\u00eancia que a eleva\u00e7\u00e3o do n\u00edvel do mar n\u00e3o ser\u00e1 a mesma em todos os lugares. O AR4 mostra um mapa de mudan\u00e7as regionais do n\u00edvel do mar, o qual mostra que por exemplo a costa europ\u00e9ia pode esperar uma eleva\u00e7\u00e3o de 5-15 cm a mais que a m\u00e9dia global de eleva\u00e7\u00e3o \u2013 isso \u00e9 uma m\u00e9dia de modelo, n\u00e3o incluindo a incerteza do intervalo. O padr\u00e3o nesse mapa \u00e9 marcadamente similar ao que seria esperado de uma desacelera\u00e7\u00e3o da na circula\u00e7\u00e3o termohalina (veja Levermann et al. 2005<\/a>) de modo que provavelmente a eleva\u00e7\u00e3o seja dominada por esse efeito. Al\u00e9m disso, algumas \u00e1reas terrestres est\u00e3o surgindo e outras desaparecendo em resposta ao final da \u00faltima era glacial ou devido \u00e0 processos antropog\u00eanicos locais (como o uso de \u00e1guas subterr\u00e2neas), os quais os gerentes e tomadores de decis\u00e3o devem tamb\u00e9m considerar.<\/p>\n

A \u00faltima linha<\/strong><\/a><\/p>\n

A principal conclus\u00e3o dessa an\u00e1lise \u00e9 que a incerteza do n\u00edvel do mar n\u00e3o \u00e9 menor agora que na \u00e9poca do TAR, e citar o intervalo de 18-59 cm para a eleva\u00e7\u00e3o do n\u00edvel do mar, como muitos artigos da m\u00eddia t\u00eam feito, n\u00e3o representa toda a est\u00f3ria. 59 cm n\u00e3o \u00e9 infortunadamente o \u201cpior caso\u201d. Ele n\u00e3o inclui toda a incerteza das placas de gelo, a qual deveria adicionar 20 cm ou mais. Ele n\u00e3o cobre totalmente o ‘prov\u00e1vel’ intervalo de temperatura dado no AR4 (at\u00e9 6.4 \u00baC) \u2013 corre\u00e7\u00f5es nesse sentido poderiam adicionar novamente cerca de 20<\/s> 15 cm. Ele n\u00e3o considera o fato de que a eleva\u00e7\u00e3o passada do n\u00edvel do mar seja subestimada pelos modelos por raz\u00f5es que s\u00e3o pouco claras. Considerando essas quest\u00f5es, uma eleva\u00e7\u00e3o do n\u00edvel do mar que exceda um metro pode, no meu ponto de vista, de modo algum ser descartada. Numa an\u00e1lise muito diferente<\/a>, baseada somente numa simples correla\u00e7\u00e3o da eleva\u00e7\u00e3o do n\u00edvel do mar e temperatura, eu cheguei a uma conclus\u00e3o similar. Como citado nesse paper, meu ponto aqui n\u00e3o <\/em> \u00e9 que eu tenha previsto que o n\u00edvel do mar ser\u00e1 maior que o IPCC sugere, ou que as estimativas do IPCC para a eleva\u00e7\u00e3o do n\u00edvel do mar n\u00e3o estejam corretas. Meu ponto \u00e9 que em termos de an\u00e1lise de risco, o intervalo de incerteza que algu\u00e9m precisa considerar \u00e9 na minha vis\u00e3o substancialmente maior que os 18-59 cm.<\/p>\n

Um pensamento final: esta discuss\u00e3o tem sido sobre a eleva\u00e7\u00e3o do n\u00edvel do mar at\u00e9 o ano de 2095. E tal eleva\u00e7\u00e3o n\u00e3o termina nesse ano, como mostra a cita\u00e7\u00e3o do SPM no in\u00edcio desse artigo. Ao longo de muitos s\u00e9culos, sem esfor\u00e7os s\u00e9rios de mitiga\u00e7\u00e3o podemos esperar muitos metros de eleva\u00e7\u00e3o dos oceanos. O Conselho Consultivo em Mudan\u00e7a Global<\/a> do governo alem\u00e3o (elucidando: sou membro desse conselho) em seu recente relat\u00f3rio especial sobre oceanos<\/a> tem proposto limitar a eleva\u00e7\u00e3o do n\u00edvel do mar a um m\u00e1ximo de um metro, como sendo uma meta a guiar a pol\u00edtica clim\u00e1tica. Mas isso \u00e9 uma outra est\u00f3ria.<\/p>\n

Atualiza\u00e7\u00e3o:<\/strong> Fui rec\u00e9m informado por um dos autores do IPCC que os cen\u00e1rios de intervalo de temperatura sem o feedback do ciclo do carbono varia at\u00e9 to 5.2 \u00baC, e n\u00e3o 4.5 \u00baC como pensava. Este n\u00famero n\u00e3o \u00e9 encontrado no relat\u00f3rio do IPCC; tentei interpret\u00e1-lo de um gr\u00e1fico, mas n\u00e3o exato o suficiente. Minhas desculpas! Os n\u00fameros no texto acima devem ser corrigidos e est\u00e3o marcados. -stefan<\/p>\n

traduzido por Ivan B. T. Lima e Fernando M. Ramos<\/a> <\/small><\/p>\n

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The sea level rise numbers published in the new IPCC report (the Fourth Assessment Report, AR4) have already caused considerable confusion. Many media articles and weblogs suggested there is good news on the sea level issue, with future sea level rise expected to be a lot less compared to the previous IPCC report (the Third […]<\/p>\n","protected":false},"author":10,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[12,1,23],"tags":[],"class_list":{"0":"post-427","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-arctic-and-antarctic","7":"category-climate-science","8":"category-ipcc","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/427","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/10"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=427"}],"version-history":[{"count":1,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/427\/revisions"}],"predecessor-version":[{"id":14532,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/427\/revisions\/14532"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=427"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=427"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=427"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}