A pause in global warming

Terrestrial datasets [1] and satellite data [2] indicate that there has not been significant global climate warming during the past 16-17 years. Most global climate models project continuous substantial warming over this period [3].

The recent pause in warming is documented by the Intergovernmental Panel on Climate Change (IPCC) in its new report, Climate Change 2013 – The Physical Science Basis (also known as “The Working Group 1 Contribution to the Fifth Assessment Report of the IPCC”). The final draft of this report and a Summary for Policy Makers (SPM) are posted at www.ipcc.ch.

The SPM suggests that the pause may be due to natural variability and is not necessarily indicative of a significant change in the longer-term global warming trend. Chapter 9 of the IPCC technical report includes a detailed discussion of the discrepancy between model simulations and empirical temperature trends. The authors of Chapter 9 suggest that the “difference between simulated and observed trends could be caused by some combination of (a) internal climate variability, (b) missing or incorrect radiative forcing, and (c) model response error.” An alternative analysis emphasizing fundamental limitations of climate models is presented in Climate Change Reconsidered II: Physical Science (a report of the Nongovernmental International Panel on Climate Change; http://climatechangereconsidered.org/).  

Loehle and Scafetta [4] have suggested that natural climate variability may have been responsible in part for both the rapid warming from 1980 to 2000 and for the recent pause in warming. Since climate models were formulated with the assumption that the rapid warming from 1980 to 2000 was due to human influence, the models may have been calibrated to run “too hot,” and thus may have predicted too much warming during the recent pause. If the natural climate cycle hypothesis of Loehle and Scafetta is correct, then the trend in global temperature may remain flat for the next decade or more.

Whatever the actual causes of the recent pause in global warming, recent temperature trends are best represented by climate models based on low emission scenarios (lower than what has occurred) or with low “climate sensitivity,” or both.  “Climate sensitivity” is a measure based on energy balance considerations of how much warming would be expected in response to “climate forcing” by increasing concentrations of greenhouse gases in the atmosphere, commonly reported in terms of °C of warming per doubling of atmospheric CO2 (or equivalent).

The IPCC SMP states that “Equilibrium climate sensitivity is likely in the range of 1.5°C and 4.5°C.” Rates of warming consistent with the upper part of this sensitivity range would have significant economic and ecological implications.  However, Lewis [5] argues that recent empirical studies support an estimate of equilibrium climate sensitivity of about 2°C. If Lewis is correct, the increase in average global temperature from now until 2100 might be 1.5°C or less (i.e., a level of warming that has been characterized in the literature as largely positive for crops, ecosystems, human health, and the economy). Clearly, the ongoing debate about climate sensitivity and temperature projections has significant implications for climate and energy policy.

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