Climate Science Underscores Urgent Need to Reduce Heat-Trapping Emissions

Major developments in climate change science have been reported since the publication of the comprehensive 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC).[1]  Recent publications indicate that the consequences of climate change are already occurring at a faster pace and are of greater magnitude than the climate models used by the IPCC projected. A few of the most compelling findings are summarized below.

More CO₂ remains in the atmosphere

Human activities have pumped excessive amounts of carbon dioxide (CO2) into the atmosphere. Natural processes that absorb CO2 cannot keep up. As the ocean absorbs carbon dioxide, it becomes more acidic. This combined with increasing ocean temperatures, diminishes its ability to continue absorbing CO2. As a result, more CO2 stays in the atmosphere. In 1960, a metric ton (1,000 kilograms; ~2,205 pounds) of CO2 emissions resulted in around 400 kilograms (~881 pounds) of CO2 remaining in the atmosphere. In 2006, a metric ton of CO2 emissions results in around 450 kilograms (~992 pounds) remaining in the atmosphere.[2] Hence a ton of CO2 emissions today results in more heat-trapping capacity in the atmosphere than the same ton emitted decades ago.

Increased sea level rise

Increased contributions from melting mountain glaciers and ice sheets on land, as well as thermal expansion due to continued ocean warming, are resulting in higher sea level rise. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) noted that sea level has risen 50 percent faster than projected by models for the 1963–2001 period. Recent observations confirm that sea level rise is in the upper range projected by models used by the IPCC (Figure 2).[3]

Plummeting Arctic sea ice

Arctic sea ice area models used by the IPCC are in general agreement with the observed area decrease over the last 50 years and indicate that heat-trapping gases are a major factor in the decline. Current observations show a much steeper drop in ice area than expected.[6] Global warming and natural cycles combine to create the observed Arctic sea ice trend. When sea ice would naturally rebound, global warming limits the full sea ice area achieved. When sea ice naturally would be less extensive in area, global warming exacerbates this natural tendency and contributes to sea ice plummeting. For example, the atmospheric pressure and wind patterns in 2007 have naturally occurred in a similar fashion at various times in decades past.  However, this type of weather pattern occurring after several decades of ice thinning combined to create a record breaking lowest summer sea ice area since satellite observations began.[7] Recent evidence shows that periods of rapid Arctic sea ice loss lead to faster warming over land in the polar region.[8] As sea ice retreats it exposes dark ocean, which absorbs more of the sun’s heat than white ice. Toward the end of summer this ocean heat dissipates to the atmosphere as the region enters winter and the ocean freezes again into sea ice. This warmer air extends over land and allows bacteria more time to decompose thawing plant and other organic matter that had been long frozen.[9] This process can lead to a release of heat-trapping gases (CO2 and methane) into the atmosphere, amplifying global warming.

CO₂ effects will be felt for generations

Studies indicate that even after excess human-caused CO2 emissions stop, the planet will experience the resulting warming for at least a thousand years. The higher the peak of atmospheric concentrations of CO2, the greater is the level of irreversible consequences, such as species loss and sea level rise.[10] These and other peer-reviewed studies published since the release of the IPCC (AR4) provide ever more compelling evidence that swift and deep reductions of heat-trapping gasses are needed if we are to avoid catastrophic climate change. United States leadership is essential, and there is no time to waste.

 References

[1] More than 450 lead authors, 800 contributing authors, and an additional 2,500 reviewing experts from more than 130 countries contributed to AR4.

[2] Canadell, J.G., C. Le Quéré, M. R. Raupach, C. B. Field, E. T. Buitenhuis, P. Ciais, T. J. Conway, N. P. Gillett, R. A. Houghton, and G. Marland. 2007. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks, Proceedings of the National Academy of Sciences.

[3] Rahmstorf, S., A. Cazenave, J.A. Church, J. E. Hansen, R. F. Keeling, D. E. Parker, R. C. J. Somerville  2007. Recent Climate Observations Compared to Projections, Science, 316:709

[4] Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

[5] W. T. Pfeffer, J. T. Harper,  S. O’Neel. 2008. Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise. Science 321:1340-1343.

[6] Stroeve, J.  Marika M. Holland, Walt Meier, Ted Scambos, and Mark Serreze (2007) Arctic sea ice decline: Faster than forecast Geophysical Research Letters, Vol. 34, L09501, Doi: 10.1029/2007gl029703

[7] Stroeve, J.,  M. Serreze, S. Drobot, S. Gearheard, M. Holland, J. Maslanik, W. Meier, And T. Scambos. 2008.  Arctic Sea Ice Area Plummets in 2007, Eos, Transactions, American Geophysical Union, 89:13-20.

[8] Serreze, M.C., A. P. Barrett, J. C. Stroeve, D. N. Kindig, and M. M. Holland. 2009. The emergence of surface-based Arctic amplification, The Cryosphere, 3:11–19.

[9] Lawrence, D.M.,  A. G. Slater, R. A. Tomas, M. M. Holland, and C. Deser. 2008. Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss, Geophysical Research Letters, 35, L11506, doi:10.1029/2008GL033985

[10] Solomon, S., G-K Plattner, R. Knutti, and P. Friedlingstein. 2009. Irreversible climate change due to carbon dioxide emissions, Proceedings of the National Academy of Sciences, 106: 1704–1709.

This summary, drafted by B. Ekwurzel of the Union of Concerned Scientists (UCS), benefited from helpful reviews by J. Canadell (CSIRO Marine and Atmospheric Research), S. Rahmstorf (Potsdam Institute for Climate Impact Research), W.T. Pfeffer (Institute of Arctic and Alpine Research, University of Colorado at Boulder), J. Harper (University of Montana), J.C. Stroeve (NSIDC University of Colorado), N. Cole, P. Frumhoff, A. Huertas, L.M. Perera, L. Shultz, and E. Spanger-Siegfried (UCS). The information contained herein is the sole responsibility of UCS.