A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate

By Climate Work Group: John Christy, Ph.D., Judith Curry, Ph.D., Steven Koonin, Ph.D., Ross McKitrick, Ph.D., Roy Spencer, Ph.D.

EXECUTIVE SUMMARY:
This report reviews scientific certainties and uncertainties in how anthropogenic carbon dioxide (CO2)
and other greenhouse gas emissions have affected, or will affect, the Nation’s climate, extreme weather
events, and selected metrics of societal well-being. Those emissions are increasing the concentration of
CO2 in the atmosphere through a complex and variable carbon cycle, where some portion of the additional
CO2 persists in the atmosphere for centuries.
Elevated concentrations of CO2 directly enhance plant growth, globally contributing to “greening” the
planet and increasing agricultural productivity [Section 2.1, Chapter 9]. They also make the oceans less
alkaline (lower the pH). That is possibly detrimental to coral reefs, although the recent rebound of the Great
Barrier Reef suggests otherwise [Section 2.2].
Carbon dioxide also acts as a greenhouse gas, exerting a warming influence on climate and weather
[Section 3.1]. Climate change projections require scenarios of future emissions. There is evidence that
scenarios widely-used in the impacts literature have overstated observed and likely future emission trends
[Section 3.1].
The world’s several dozen global climate models offer little guidance on how much the climate
responds to elevated CO2, with the average surface warming under a doubling of the CO2 concentration
ranging from 1.8°C to 5.7°C [Section 4.2]. Data-driven methods yield a lower and narrower range [Section
4.3]. Global climate models generally run “hot” in their description of the climate of the past few decades
− too much warming at the surface and too much amplification of warming in the lower- and mid
troposphere [Sections 5.2-5.4]. The combination of overly sensitive models and implausible extreme
scenarios for future emissions yields exaggerated projections of future warming.
Most extreme weather events in the U.S. do not show long-term trends. Claims of increased frequency
or intensity of hurricanes, tornadoes, floods, and droughts are not supported by U.S. historical data [Sections
6.1-6.7]. Additionally, forest management practices are often overlooked in assessing changes in wildfire
activity [Section 6.8]. Global sea level has risen approximately 8 inches since 1900, but there are significant
regional variations driven primarily by local land subsidence; U.S. tide gauge measurements in aggregate
show no obvious acceleration in sea level rise beyond the historical average rate [Chapter 7].
Attribution of climate change or extreme weather events to human CO2 emissions is challenged by
natural climate variability, data limitations, and inherent model deficiencies [Chapter 8]. Moreover, solar
activity’s contribution to the late 20th century warming might be underestimated [Section 8.3.1].
Both models and experience suggest that CO2-induced warming might be less damaging economically
than commonly believed, and excessively aggressive mitigation policies could prove more detrimental than
beneficial [Chapters 9, 10, Section 11.1]. Social Cost of Carbon estimates, which attempt to quantify the
economic damage of CO2 emissions, are highly sensitive to their underlying assumptions and so provide
limited independent information [Section 11.2].
U.S. policy actions are expected to have undetectably small direct impacts on the global climate and
any effects will emerge only with long delays [Chapter 12].

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