Sometimes journalists are so focused on a particular story that they ‘hear what they want to hear and disregard the rest’. There was a perfect example of this last week in the Guardian reporting from the RAPID Climate Change conference in Birmingham (UK) which I was attending. The conference, whose theme was observations, modelling and paleo-climate related to the Thermohaline and Meridional overturning circulation (MOC) in the North Atlantic, could have been expected to attract media attention (particularly in the Europe) and indeed it did. However, the Guardian story, which started “Scientists have uncovered more evidence for a dramatic weakening in the vast ocean current that gives Britain its relatively balmy climate” was in complete opposition to the actual evidence presented and I wasn’t the only person to notice. How could the reporting be so wrong?
[Read more…] about Ocean Circulation: New evidence (Yes), slowdown (No)
Oceans
Carl Wunsch, The Economist and the Gulf Stream
Carl Wunsch usually has very interesting things to say about the climate system, and although his arguments don’t necessarily win everyone completely over, they often generate an improvement in the level of scientific discussion. In this week’s Economist, he has a letter printed concerning the mis-definition of the ‘Gulf Stream’ concept in the magazine’s climate change survey a couple of weeks ago. This is essentially a reprint of his letter to Nature that was published in 2004, which stated correctly that the Gulf Stream is basically a wind driven phenomenon and will not stop or reverse while the wind still blows and the Earth still turns. [Read more…] about Carl Wunsch, The Economist and the Gulf Stream
Why greenhouse gases heat the ocean
Guest commentary by Peter Minnett (RSMAS)
Observations of ocean temperatures have revealed that the ocean heat content has been increasing significantly over recent decades (Willis et al, 2004; Levitus et al, 2005; Lyman et al, 2006). This is something that has been predicted by climate models (and confirmed notably by Hansen et al, 2005), and has therefore been described as a ‘smoking gun’ for human-caused greenhouse gases.
However, some have insisted that there is a paradox here – how can a forcing driven by longwave absorption and emission impact the ocean below since the infrared radiation does not penetrate more than a few micrometers into the ocean? Resolution of this conundrum is to be found in the recognition that the skin layer temperature gradient not only exists as a result of the ocean-atmosphere temperature difference, but also helps to control the ocean-atmosphere heat flux. (The ‘skin layer‘ is the very thin – up to 1 mm – layer at the top of ocean that is in direct contact with the atmosphere). Reducing the size of the temperature gradient through the skin layer reduces the flux. Thus, if the absorption of the infrared emission from atmospheric greenhouse gases reduces the gradient through the skin layer, the flow of heat from the ocean beneath will be reduced, leaving more of the heat introduced into the bulk of the upper oceanic layer by the absorption of sunlight to remain there to increase water temperature. Experimental evidence for this mechanism can be seen in at-sea measurements of the ocean skin and bulk temperatures. [Read more…] about Why greenhouse gases heat the ocean
Ocean heat content: latest numbers
Net ocean heat content changes are very closely tied to the net radiative imbalance of the planet since the ocean component of the climate system has by far the biggest heat capacity. Thus we have often made the point that diagnosing this imbalance through measurements of temperature in the ocean is a key metric in evaluating the response of the system to changes in CO2 and the other radiative forcings (see here).
In a paper I co-authored last year (Hansen et al, 2005), we compared model results with the trends over the 1993 to 2003 period and showed that they matched quite well (here). Given their importance in evaluating climate models, new reports on the ocean heat content numbers are anticipated quite closely.
Recently, a new preprint with the latest observations (2003 to 2005) has appeared (Lyman et al, hat tip to Climate Science) which shows a decrease in the ocean heat content over those two years, decreasing the magnitude of the long-term trend that had been shown from 1993 to 2003 in previous work (Willis et al, 2004) – from 0.6 W/m2 to about 0.33 W/m2. This has generated a lot of commentary in some circles, but in many cases the full context has not been appreciated.
[Read more…] about Ocean heat content: latest numbers
Sea level in the Arctic
A recent conference presentation at AGU (reported here) while confirming that global sea level is indeed rising (in line with other estimates), showed that Arctic sea levels may actually be falling. On the face of it these preliminary results are a little puzzling (though note that this isn’t yet a properly peer reviewed paper, and so may not reflect what ends up in the journal), but it does reveal some of the complexities in analysing sea level in relatively small enclosed basins and so a brief overview of the different factors involved is probably useful. [Read more…] about Sea level in the Arctic
On a Weakening of the Walker Circulation
by Ray Pierrehumbert and Rasmus Benestad
Second article of our 3-part series on atmospheric circulation and global warming
In Part I we outlined some general features of the tropical circulation, and discussed ways in which increases in anthropogenic greenhouse gases might affect El Niño. Now we take up the question of how global warming might affect the quasi-steady east-west overturning circulation known as the Walker Circulation. The Walker circulation affects convection and precipitation patterns, the easterly Trade Winds, oceanic upwelling and ocean biological productivity; hence, changes in this circulation can have far-reaching consequences. It also provides the background state against which El Niño events take place, and so changes in the Walker circulation should form an intrinsic part of thinking about how global warming will affect El Niño. In a paper that recently appeared in Nature, Vecchi, Soden, Wittenberg, Held, Leetmaa and Harrison present intriguing new results which suggest that there has already been a weakening of the Walker circulation in the past century, and that the observed changes are consistent with those expected as a response to increases in anthropogenic greenhouse gases. The discussion in Vecchi et al. also raises some very interesting issues regarding the way the hydrological cycle might change in a warming world.
El Niño and Global Warming
By Rasmus Benestad & Raymond Pierrehumbert
This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.
Sir Nicholas Shackleton
With the recent death of Sir Nicholas Shackleton, paleoclimatology lost one of its brightest pioneers. Over the last ~40 years, Nick made numerous far-reaching contributions to our understanding of how climates varied in the past, and through those studies, he identified factors that are critically important for climate variability in the future. His career neatly encompasses the birth of the new science of paleoceanography to its synthesis into the even newer science of ‘Earth Systems’; he made major contributions to these evolving fields throughout his life, and his insightful papers are required reading for students of paleoclimatology.
[Read more…] about Sir Nicholas Shackleton
Atlantic circulation change summary
Nature this week has an excellent summary of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it’s still worth reading if you want to understand how results like the Bryden et al paper – that suggested that the Atlantic overturning had reduced by 30% in recent decades – are assimilated into the scientific picture. [Read more…] about Atlantic circulation change summary
Was the record Amazon drought caused by warm seas?
On December 11, 2005, The New York Times ran a story on record drought conditions in the Amazonas region of Brasil, linking it to global warming, and specifically the warm ocean temperatures in the North Atlantic that have also been linked to the ferocity of the 2005 Atlantic hurricane season. This prompted a response from Chris Mooney, calling for a comment from RealClimate about whether such an assertion is valid, as we earlier made it very clear that it is impossible to say whether one single extreme event in a very noisy environment – such as Hurricane Katrina – is related to climate change. So we decided to take a look at this phenomena, and address why there might be a connection and what it takes to make an attribution. [Read more…] about Was the record Amazon drought caused by warm seas?