Intergovernmental Panel on Climate Change releases fifth climate change assessment summary

The newest Intergovernmental Panel on Climate Change (IPCC) assessment release confirms recent global warming trends and suggests additional warming in the future.

The Intergovernmental Panel on Climate Change (IPCC) is an international body of scientists established by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) to provide periodic assessments of the current state of knowledge in climate change and its potential environmental and socio-economic impacts. Containing the results of current research from thousands of scientists around the world, it is the most detailed and comprehensive report of its kind. Last Friday, Sept. 27, 2013, the IPCC issued a summary statement concerning its new Fifth Assessment Report (AR5), which is being released in four parts between September 2013 and November 2014.

The results of the summary were similar to the last IPCC Assessment Report (AR4) released in 2007 with a few notable changes. Overall, the summary concludes that the earth’s climate system, including the atmosphere and oceans, has warmed during the past century and that since the 1950s, many of the observed changes are unprecedented over time periods of tens to thousands of years.

For example, the 1983–2012 period in the Northern Hemisphere was likely the warmest 30-year period for that part of the world in at least 1,400 years. Almost all regions of the world warmed during the 1901-2012 period. The total global increase in temperature between the 1850–1900 period and the 2003–2012 period was 1.4 degrees Fahrenheit; for comparison, the change in mean annual temperature in Michigan from 1900 through the present has been about 0.5 F.

The IPCC provides a degree of certainty, or uncertainty, with their conclusions based on the collective research evidence. In the case of a warming climate system, the term used was “unequivocal.” The report also noted changes in the frequency and magnitude of some types of extreme weather events since 1950, including decreases in the number of cold days and increases in the number of warm days and nights, heat waves and heavy precipitation events.

While the earth’s climate system is dynamic by nature, there is strong evidence that human activities are at least partially responsible for recent warming. Increasing concentrations of greenhouse gasses have led to a positive forcing of the earth’s energy balance with relatively less energy leaving the system into space over time. Regarding these changes, the AR5 report states, “Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming and understanding of the climate system…Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes.”

Simulations of historical and projected future climate with comprehensive global climate models strengthen the link between human activities and recent trends and suggest additional changes in the future. The report concludes that, “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.”

Historical and projected future mean global temperature changes with time are illustrated in Figure 1. There were four major greenhouse gas emission scenarios used for the future timeframe ranging from a with high emission rate “business as usual” scenario (RCP8.5 in red) to a high conservation/new mitigation technology scenario with an eventual decline in emissions (RCP2.6 in purple). The overall increases in mean global temperature for the four scenarios range from about 1.0 to 8.0 F. While the rates of warming vary significantly by scenario, they are still almost all greater than the historical changes. As to how we should respond, the report concludes that effective options are limited, and that, “Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”

Figure 1
Figure 1. Simulated time series from multiple global climate models from 1950 to 2100 depicting the change in global annual mean surface temperature (degrees Celsius) relative to the 1986–2005 period. Future projections and a measure of uncertainty (depicted by the shading) are shown for low (RCP2.6 in blue) and high (RCP8.5 in red) emission scenarios. Black (grey shading) is the modeled historical evolution using historical reconstructed forcings. The mean temperature change and associated uncertainties averaged over the 2081−2100 period are given on the right for all emission scenarios as colored vertical bars. The numbers of global climate models used to calculate the multi-model mean are indicated next to the traces. Figure courtesy of IPCC

As noted earlier, the conclusions of the report are in general similar to those of past Assessment Reports. One change is the degree of scientific certainty of some of the observations and projections. For example, with respect to the human influence on the warming, the AR5 report states, “It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.” The “extremely likely” term suggests 95 to 100 percent confidence in the statement and is relatively greater than previous reports. Another way of thinking of this is that the uncertainty associated with many aspects of climate change science has decreased with time.

The current report also more strongly emphasizes the risks associated with sea level rise, which has occurred relatively more quickly than previously expected. While it does not directly affect much of Michigan’s population, sea level rise threatens more than one billion people around the world living in low-lying coastal communities (i.e., witness the impacts of Superstorm Sandy on the East Coast last fall) and may be the most expensive of all climate change-related threats. The AR5 report concludes that sea level rose almost twice as fast from 1993-2010 as from 1901-2010, and that under all future projected scenarios, “The rate of sea level rise will very likely exceed the rates observed during 1971-2010.”

Another interesting conclusion regards the slowing of the increase in mean global temperatures during the past 15 years (relative to the past few decades). The rate of warming over the 1998–2012 was only 0.1 F per decade, while during the 1951-2012 period it averaged 0.2 F per decade. In response to this observation, the report states that at least some of the recent slowing is due to substantial decadal and interannual variability possibly associated with large atmospheric or oceanic cycles; 1998 was a strong El Niño year, which is typically associated with above normal temperatures. More importantly, the warming due to large scale greenhouse gas forcing continued during the period with the vast majority – more than 90 percent – of the additional energy being absorbed by the oceans. Recent research suggests that the recent energy partitioning pattern is very likely temporary, with a resumption of increasing atmospheric temperatures probable in the future.

Read the entire IPCC Fifth Assessment Report Summary for Policymakers.

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