RealClimate logo


O Say Can You CO2…

Filed under: — group @ 12 October 2017

Guest Commentary by Scott Denning

The Orbiting Carbon Observatory (OCO-2) was launched in 2014 to make fine-scale measurements of the total column concentration of CO2 in the atmosphere. As luck would have it, the initial couple of years of data from OCO-2 documented a period with the fastest rate of CO2 increase ever measured, more than 3 ppm per year (Jacobson et al, 2016;Wang et al, 2017) during a huge El Niño event that also saw global temperatures spike to record levels.

As part of a series of OCO-2 papers being published this week, a new Science paper by Junjie Liu and colleagues used NASA’s comprehensive Carbon Monitoring System to analyze millions of measurements from OCO-2 and other satellites to map the impact of the 2015-16 El Niño on sources and sinks of CO2, providing insight into the mechanisms controlling carbon-climate feedback.

Uncertainty in Carbon-Climate Feedbacks is important

We’ve known for decades (Rayner et al, 1999) that El Niño influences the productivity of tropical forests and therefore CO2, but we had very few direct observations of the effects because they are so remote. Field experiments on the ground and aircraft profiling of CO2 over tropical forests have documented the impact of heat and drought on forest productivity, but they are few and far between. Vigorous convective mixing in the deep tropics also dilutes changes in near-surface CO2 much more than at higher latitudes, so low-altitude sampling contains relatively less information about carbon sources and sinks.

A subset of Earth System Models (ESMs) project that El Niño-like conditions will progressively increase in coming decades as sea-surface temperatures in the tropical Pacific warm, implying increased drought and forest dieback in the Amazon. The drought-induced decline of carbon-dense tropical forests and their replacement by lower-carbon savannas would release enormous amounts of CO2 to the atmosphere, amplifying global warming far beyond the effects of just the CO2 released by burning fossil fuels. In the CMIP5 suite of ESMs summarized by the IPCC Fifth Assessment Report, models forced with identical fossil fuel emissions differed by as much as 350 ppm of CO2 in 2100 due to differences in feedback between climate and the carbon cycle (Hoffmann et al, 2014). The radiative forcing of climate in these ESMs differed by up to 1.5 W/m2, with much of the disparity being driven by interactions among warming oceans, atmospheric circulation, and tropical forests. The climate outcomes due to differences carbon-climate feedback are as different as those arising from different future emission scenarios (RCP6 compared to RCP4.5) or from differences in clouds and aerosols in atmospheric models.

The NASA Carbon Monitoring System

NASA’s Carbon Monitoring System (CMS) combines mechanistic “forward” models and empirical “inverse” models of atmospheric CO2 and other variables using a technique called “data assimilation” that is closely analogous to operational weather forecasting (Bowman et al, 2017). The forward models include emissions of CO2 and carbon monoxide (CO) from fossil fuel burning and wildfires; air-sea gas exchange; and photosynthesis, respiration, and decomposition on land. These simulated emissions are then used as input to a model (GEOS-Chem) that uses high-resolution weather data atmospheric transport of CO2 and CO by winds, clouds, and turbulence. The resulting 3D simulations are then sampled at the locations of OCO-2 observations to determine the error in the forward model of atmospheric variations. An “adjoint” of the atmospheric transport model is then run backward in time to quantify the contributions of errors in specified surface sources and sinks of CO2 and CO to the mismatches between the forward models and the satellite observations.

OCO-2 has given us two revolutionary new ways to understand the effects of drought and heat on tropical forests. The instrument directly measures CO2 over these regions thousands of times every day (Crisp et al, 2004). These column-averaged concentration retrievals respond to the net amount of CO2 passing in and out of the atmosphere under the instrument. OCO-2 also senses the rate of photosynthesis by detecting fluorescent chlorophyll in the trees themselves (Frankenberg et al, 2011). Liu et al used observations of CO from the MOPITT instrument aboard NASA’s Terra satellite to identify CO2 released from upwind wildfires. They used solar-induced chlorophyll fluorescence (SIF) to quantify changes in plant photosynthesis (also called gross primary production, GPP). Their results include time-resolved maps of the sources and sinks of atmospheric CO2 that are optimally consistent with both mechanistic forward models and the CO2, CO, and SIF observed by the satellite instruments.




Fig. Extreme heat and drought impacted the carbon cycle in tropical forests differently in different regions, leading to the fastest growth rate of CO2 in at least 10,000 years. (NASA/JPL-Caltech).

Carbon-Climate Feedback During the 2015-16 El Niño

As previously reported based on in-situ data, the rate of increase in atmospheric CO2 during the strong El Niño in 2015-16 was about 3 ppm/yr compared with ~2 ppm/yr in recent decades. This is the fastest increase in CO2 ever observed, and plausibly the fastest since the end of deglaciation 10,000 years ago. Yet this rapid increase in CO2 occurred during a period when fossil fuel emissions were nearly flat (though still massively more than the biosphere and ocean can quickly absorb).

Liu et al found that 80% of the extra CO2 in the atmosphere during this period originated in tropical forests. Relative to a more normal year (2011), they found that tropical forests lost about 2.5 billion tons of carbon (GtC) in 2015-16. (1 Gt = 1012 kg is the mass of 1 cubic km of water, and 1 GtC produces about 2.12 ppm of CO2 in the air).

During the huge El Niño, parts of the Amazon experienced the driest conditions in at least 30 years as well as unusually warm temperatures. Changes in column-averaged CO2 and in SIF showed that these hot, dry conditions suppressed gross primary production (GPP, photosynthesis), leading to a reduction of about 0.9 GtC/yr. Equatorial Africa also experienced extreme heat, but precipitation was near normal. Impacts on GPP were not significant, but respiration and decomposition were enhanced by about 0.8 GtC/yr. In Hot dry conditions in Indonesia during the period led to an increase in fires, including a large peat fire that burned huge amounts of stored carbon. Emissions due to these fires showed up in the observations as increases in both CO2 and CO, and were estimated at about 0.8 GtC/yr.

These results help us understand how drought and heat affect these forests, some of the most productive ecosystems on Earth. The Amazon has experienced three extreme droughts in the past 11 years, in 2005, 2010, and now 2015-16. These extreme events have occurred more frequently than they did in the previous century. Understanding how the tropical forest responds to big droughts and heat waves help us to evaluate the strength of carbon-climate feedback in ESMs, allowing us to better understand and predict climate change over coming decades. The new results show that each of the major tropical forest regions experienced different combinations of heat and drought during the recent El Niño, so their carbon cycles responded in different ways, but the net result was increased emissions in all cases. Based on these results, further warming and drying of tropical forests is expected to result in less uptake and more release of carbon on land, unfortunately amplifying the effect of fossil fuel emissions warming the climate.

References

  1. J. Wang, N. Zeng, M. Wang, F. Jiang, H. Wang, and Z. Jiang, "Contrasting terrestrial carbon cycle responses to the two strongest El Niño events: 1997–98 and 2015–16 El Niños", Earth System Dynamics Discussions, pp. 1-32, 2017. http://dx.doi.org/10.5194/esd-2017-46
  2. J. Liu, K.W. Bowman, D.S. Schimel, N.C. Parazoo, Z. Jiang, M. Lee, A.A. Bloom, D. Wunch, C. Frankenberg, Y. Sun, C.W. O’Dell, K.R. Gurney, D. Menemenlis, M. Gierach, D. Crisp, and A. Eldering, "Contrasting carbon cycle responses of the tropical continents to the 2015–2016 El Niño", Science, vol. 358, pp. eaam5690, 2017. http://dx.doi.org/10.1126/science.aam5690
  3. P.J. Rayner, R.M. Law, and R. Dargaville, "The relationship between tropical CO2fluxes and the El Niño-Southern Oscillation", Geophysical Research Letters, vol. 26, pp. 493-496, 1999. http://dx.doi.org/10.1029/1999GL900008
  4. "Climate Change 2013 - The Physical Science Basis", 2009. http://dx.doi.org/10.1017/CBO9781107415324
  5. K.W. Bowman, J. Liu, A.A. Bloom, N.C. Parazoo, M. Lee, Z. Jiang, D. Menemenlis, M.M. Gierach, G.J. Collatz, K.R. Gurney, and D. Wunch, "Global and Brazilian carbon response to El Niño Modoki 2011-2010", Earth and Space Science, 2017. http://dx.doi.org/10.1002/2016EA000204
  6. D. Crisp, R. Atlas, F. Breon, L. Brown, J. Burrows, P. Ciais, B. Connor, S. Doney, I. Fung, D. Jacob, C. Miller, D. O'Brien, S. Pawson, J. Randerson, P. Rayner, R. Salawitch, S. Sander, B. Sen, G. Stephens, P. Tans, G. Toon, P. Wennberg, S. Wofsy, Y. Yung, Z. Kuang, B. Chudasama, G. Sprague, B. Weiss, R. Pollock, D. Kenyon, and S. Schroll, "The Orbiting Carbon Observatory (OCO) mission", Advances in Space Research, vol. 34, pp. 700-709, 2004. http://dx.doi.org/10.1016/j.asr.2003.08.062
  7. C. Frankenberg, C. O'Dell, J. Berry, L. Guanter, J. Joiner, P. Köhler, R. Pollock, and T.E. Taylor, "Prospects for chlorophyll fluorescence remote sensing from the Orbiting Carbon Observatory-2", Remote Sensing of Environment, vol. 147, pp. 1-12, 2014. http://dx.doi.org/10.1016/j.rse.2014.02.007

30 Responses to “O Say Can You CO2…”

  1. 1
    patrick says:

    > This is the fastest increase in CO2 ever observed, and plausibly the fastest since the end of deglaciation 10,000 years ago. Yet this rapid increase in CO2 occurred during a period when fossil fuel emissions were nearly flat (though still massively more than the biosphere and ocean can quickly absorb). …Based on these results, further warming and drying of tropical forests is expected to result in less uptake and more release of carbon on land, unfortunately amplifying the effect of fossil fuel emissions warming the climate.

    Thank you for this super-informative post. The explanations and significance given are very clear.

  2. 2
    t marvell says:

    It looks more and more like reducing CO2 emissions as planned by the Paris agreement is grossly inadequate.

  3. 3
    james says:

    Let’s posit 3 carbon compartments whose dynamics we must quantify ASAP
    1. The tropics – and the response (releasing C? how much?)
    2. The NH temperate and boreal forests (will they continue to be a sink? how much?)
    3. The permafrost plus the Arctic continental shelves (C as both CO@ and methane? how much?)

  4. 4
    prokaryotes says:

    “Understanding how the tropical forest responds to big droughts and heat waves help us to evaluate the strength of carbon-climate feedback.. [] ..further warming and drying of tropical forests is expected to result in less uptake and more release of carbon on land..”

    Soil carbon feedback https://en.wikipedia.org/wiki/Soil_carbon_feedback

  5. 5
    d helfrich says:

    Agreed, TM. But I think we have to expect that the Paris agreement will evolve as the seriousness of our global predicament grows on the public (and on the corporations and governing bodies that have such an undue influence in energy supply matters.) On a sociological/psychological plane, once enough potential arises for change, behaviors and events can quite rapidly shift to fit the new paradigm, such that we collectively often look back and feel rather amazed at how fast the final steps in the transition occurred. (I wonder if the RCPs can fully account for this kind of non-linearity.)

    I hope to soon see some momentous steps towards resolving the uncertainties remaining in the science of how water vapor, aerosols, and clouds are related to the bigger picture. And hopefully, arising from that, our institutions and peoples will finally be ready for that rapid transition that’s so overdue. We need to expect upheavals, but on the other hand, we have to hope that we do not greatly damage our collective ability to adapt to the baked-in warming that’s to come.

  6. 6
  7. 7
    Thomas says:

    OCO-2 also senses the rate of photosynthesis by detecting fluorescent chlorophyll in the trees themselves (Frankenberg et al, 2011). Liu et al used observations of CO from the MOPITT instrument aboard NASA’s Terra satellite to identify CO2 released from upwind wildfires. They used solar-induced chlorophyll fluorescence (SIF) to quantify changes in plant photosynthesis (also called gross primary production, GPP).

    WOW~! You guys (in general) are geniuses aka brilliant!

  8. 8
    Steve Bloom says:

    “Equatorial Africa also experienced extreme heat, but precipitation was near normal. Impacts on GPP were not significant, but respiration and decomposition were enhanced by about 0.8 GtC/yr.”

    Huh. Implication for CO2 fertilization?

  9. 9
    Thomas says:

    Not long now before all these dynamics in the climate system being driven by global warming from GHGs will line up almost all at once like dominoes (from the Arctic to the antarctic and across all continents and oceans).

    The da poop will really hit the fan over AGW/CC.

    btw more people dead & missing, and more houses destroyed and people evacuated in Vietnam due to record flooding than in California wild fires. Driest warmest winter in Oz. Weirdest summer weather in the Arctic, like ever. No ice left more than 3m thick. Used to be 6-8 meters thick wasn’t it?

  10. 10
    Mal Adapted says:

    It’s worth mentioning that the current issue of Science is dedicated to remote sensing for climate science. Of particular interest to North Americans west of the 100th meridian, will be a review of Firestorm, by one Edward Struzik:

    Why do wildfires happen? Are large fires becoming more common? Is climate change making things worse? How will these fires shape our future? Writer and photographer Edward Struzik illuminates these and many other questions in Firestorm. His compelling narrative combines personal stories, photographs, history, and interviews with scientists, government officials, and the public. What emerges is a portrait of the intricacies of the forest management practices, settlement patterns, economic motivators, and political pressures that lead up to and influence decisions during these wildfires.

    I just downloaded it for Kindle for US$17.00. It’s too easy to buy books online, goddammit.

  11. 11
    Eli Rabett says:

    Given this analysis, the CO2 mixing ratio should have spurted in El Nino years, and it certainly looks like it did, esp
    the strong El Nino year of 1997-1998

  12. 12
    Everett F Sargent says:

    The oceans are the largest CO2 sink, by far, yet the ocean sink gets nary a mention? How come?

    I’ve been looking at the SIO (infilled by me) weekly CO2 time series, oh say on a weekly basis for a few months now, removing the long term anthro CO2 trend, then pulling out the 1st four harmonics (well there might be five, but that one is iffy from a statistical standpoint), then running a properly constructed IIR low pass filter. It looks like 2017 will be as low as 2.2 ppmv/yr. Looks like ESRL has bottomed out, while I’ll have to wait another month to see the monthly release of the SIO data (currently at a low end point).

    The claim of two years of 3 ppmv/yr occurred because of the way the CO2 time series straddled almost perfectly those two years (2015-6) along with the long term anthro atmospheric rate of change.

    I’m seeing a virtual tie with the 1973-4, 1997-8 and 2015-6 peaking at about 2 ppmv/yr above the long term anthro atmospheric increase.

    Bye.

  13. 13
    Jef says:

    helfrich @ #5 – It always scares the heck out out of me when people talk about “that rapid transition…” as if it solves anything. A rapid transition insures that we must ramp up FF use, increase cement production exponentially, crank up all mining and refining all while maintaining the existing economy to keep from collapsing.

    What we need to do is POWER DOWN industrial civilization to a fraction of current levels then focus on making sure everyone is OK.

  14. 14

    @ Everett: You are right – when taking the anthropogenic emissions into account the recent El Nino does not look exceptional. Indeed, seen as an anomaly, the 1997-8 was even stronger. I commented on this in my blog:

    https://bgcjena.wordpress.com

  15. 15
    mike says:

    I appreciate what ES said here at 12. I think the 3.0 increases were largely about a really significant EN event that triggered release of a lot of CO2 from natural sources that stopped acting as sinks and started acting as sources under the heat bump of the large EN event.

    The CO2 increase range that I expect for 2017 is 2.4 ppm. It might be 2.2 ppm or even less. This will be a “falloff” in accumulation that is not real because the comparison of a non-EN year with a EN year is apples and oranges. Best look is at multi-year decadal rates that smooth out the bumps. You can check the decadal rates to see how we are really doing.

    Decade

    Atmospheric CO2 Growth Rate
    2005 – 2014 2.11 ppm per year
    1995 – 2004 1.87 ppm per year
    1985 – 1994 1.42 ppm per year
    1975 – 1984 1.44 ppm per year
    1965 – 1974 1.06 ppm per year
    1959 – 1964 0.73 ppm per year
    (6 years only)

    We can study this til the cows come home and we probably will, but it’s clear to any rational person that we have moved into dangerous territory with CO2 of 405 ppm and above and we actually have to move the needle down. Slowing the rate of increase of CO2 gets a lot of ink, but it’s a misleading metric if you don’t state prominently that the CO2 now needs to be negative, not just slowed on upward trend, we have to turn it the other direction.

    Nice to see this post at RC,

    Thanks

    Mike

  16. 16
    Steve Bloom says:

    Re #12: Perhaps, except the current levels incorporate (claimed) flat anthropogenic emissions. If those emissions are being replaced by feedback ones, especially from the tropical forests, it’s not at all good news. Scott?

  17. 17
    Barry says:

    I am confused
    The diagrams show 0.9, 0.8, 0.8 GtC/yr
    All plus
    But the wording says 0.9 reduction in the Amazon.
    Presumably the diagram should show -0.9 GtC/yr for the Amazon
    but then again at the end it says
    ¨but the net result was increased emissions in all cases¨
    I would appreciate any clarification.

  18. 18

    Steve Bloom #8: Implications for CO2 fertilization are one of the most important reasons to do these measurements. Other sinks too: N deposition, forest regrowth in the developed world, & longer boreal growing seasons. All of these land sink mechanisms can be impacted by climate change (carbon-climate feedback). ESMs simulate a huge variety of future trajectories of land sink, including switching to a huge source. It’s critical to test the models against real data, which is much easier when we have global high-res coverage like OCO-2.

    Many of us have been surprised at the persistence of a strong land sink for 50+ years, but substantial reduction or saturation of sinks would lead to much more rapid CO2 accumulation.

  19. 19

    Eli Rabett #11: Relationship between ENSO and CO2 growth rate has been known since at least 1976 (Bacastow et al). There’s typically an initial ocean uptake as tropical East Pacific upwelling (CO2 degassing) is reduced, followed by a stronger release of carbon from land.

    The big spike in 1998 was apparently associated with huge peat fires in Indonesia which also grounded airplanes for weeks due to smoke. Forest impacts of Amazon droughts in 2005 and 2010 were documented in situ and from airborne CO2 as well as in MODIS imagery.

    The Liu paper breaks new ground because it combines column CO2 from space (much larger N) with SIF and CO retrievals in a model to diagnose both fluxes and mechanisms. This lets us test deterministic models against real data in a natural experiment in a quantitative way that has previously been very limited by observations.

  20. 20

    Everett F Sargent #12: Ocean carbon storage is ~ 20x land storage, but average ocean sink in a given year is about the same as the average land sink.

    Chatterjee et al 2017 (http://science.sciencemag.org/content/358/6360/eaam5776) in the current issue of Science used OCO-2 and in-situ data to show that El Nino event reduced outgassing of CO2 from tropical east Pacific in 2015, leading to anomalous sink. Under normal conditions upwelling of cold CO2-rich water from depth leads to outgassing when upwelled water warms at the surface. This slows down during ENSO warm phase. Later in the record, this reduced outgassing was counteracted by the land carbon source documented by Liu et al in the paper I described here.

  21. 21

    Steve Bloom #16: Regarding whether this is “good news,” no I agree it’s not. There was a substantial carbon cycle response in tropical forests in 2015-16 to the heat and drought. Rather than see it as “good or bad,” I think of this as progress that helps us to evaluate our understanding and ability to predict future changes in land sinks.

  22. 22

    Barry #17: The results reported in the paper and the numbers on the figure show that all of the regions contributed carbon to the atmosphere relative to what they did in 2011. Hence the plus signs.

    In the Amazon, Liu et al found that the change resulted from a *decrease in photosynthesis* (as revealed by changes in chlorophyll fluorescence).

    The sign convention is positive into the atmosphere, negative out of the atmosphere. So a smaller uptake over the Amazon is a positive contribution to the atmosphere.

    Make sense?

  23. 23
    Russell says:

    19: “The big spike in 1998 was apparently associated with huge peat fires in Indonesia which also grounded airplanes for weeks due to smoke”

    This season pales in comparison to the 1984 fires set by transmigrasi– Javans treansplanted to Borneo to relieve overcrowding on their home island . Seeking to clear surface brush, they ignited fires that spread to the centuies deep peat soils that had dried out in consequence of the tropical forest clearance that preceded their arrival.

  24. 24
    Keith Woollard says:

    Thomas @#9 “Driest warmest winter in Oz”
    ‘fraid not.
    http://www.bom.gov.au/climate/current/season/aus/summary.shtml
    Fifth warmest
    Ninth driest

  25. 25
    d helfrich says:

    Barry #17: The “reduction” is in reference to the gross primary production (GPP) of vegetation, which is a carbon absorption process, or sink. A reduction a carbon sink’s intake of Carbon via photosynthesis amounts to an increase in the carbon dioxide burden in the atmosphere.

  26. 26
    Andrew O says:

    Any data on the forests in the North American Pacific Northwest?

  27. 27
    mike says:

    AO at 26: per the PNW forests. My spouse and I walk the same areas around Tahoma or Mt. Rainier year after year because some specific hikes and destinations are stunning in their natural beauty. I can tell you that some parts of these forests have suffered quite a bit from the heat and lack of moisture. This year was a little different because we had a better snowpack than for the past few, but I have little doubt that the PNW forests have struggled with the climb in global temps as that has played out on the high ground, the parks, the wilderness areas.

  28. 28
    Barry says:

    Scott Denning @22
    D helfrich @25
    Thank you

  29. 29
    calyptorhynchus says:

    “As luck would have it, the initial couple of years of data from OCO-2 documented a period with the fastest rate of CO2 increase ever measured…”

    Not very lucky!

  30. 30
    Thomas says:

    Sorry typo …. “Driest warmest winter [period] in [many parts of] Oz.” :-)

Leave a Reply

Comment policy.