Figure 4. As the climate system varies naturally from year to year and from decade to decade, reliable inferences about human-induced climate change must be made with a longer view, using multi-decadal and longer records. Calculating a ‘running average’ over these longer timescales allows one to more easily see long-term trends. For the global average temperature for the period 1850-2012 (using the data from the UK Met Office Hadley Centre relative to the 1961-90 average) the plots show: (top) the average and range of uncertainty for annually averaged data; (2nd plot) the temperature given for any date is the average for the ten years about that date; (3rd plot) the equivalent picture for 30-year; and (4th plot) the 60-year averages. Source: Met Office, based on the HadCRUT4 dataset from the Met Office and Climatic Research Unit (Morice et al., 2012). (larger version)
Decades of slow warming as well as decades of accelerated warming occur naturally in the climate system. Decades that are cold or warm compared to the long-term trend are seen in the observations of the past 150 years and also captured by climate models. Because the atmosphere stores very little heat, surface temperatures can be rapidly affected by heat uptake elsewhere in the climate system and by changes in external influences on climate (such as particles formed from material lofted high into the atmosphere from volcanic eruptions). More than 90% of the heat added to Earth is absorbed by the oceans and penetrates only slowly into deep water. A faster rate of heat penetration into the deeper ocean will slow the warming seen at the surface and in the atmosphere, but by itself will not change the long-term warming that will occur from a given amount of CO2. For example, recent studies show that some heat comes out of the ocean into the atmosphere during warm El Niño events, and more heat penetrates to ocean depths in cold La Niñas. Such changes occur repeatedly over timescales of decades and longer. An example is the major El Niño event in 1997–98 when the globally averaged air temperature soared to the highest level in the 20th century as the ocean lost heat to the atmosphere, mainly by evaporation.
Recent studies have also pointed to a number of other small cooling influences over the past decade or so. These include a relatively quiet period of solar activity and a measured increase in the amount of aerosols (reflective particles) in the atmosphere due to the cumulative effects of a succession of small volcanic eruptions. The combination of these factors, both the interaction between the ocean and the atmosphere and the forcing from the Sun and aerosols, is thought likely to be responsible for the recent slowdown in surface warming.
Despite the decadal slowdown in the rise of average surface temperature, a longer-term warming trend is still evident (see Figure 4). Each of the last three decades was warmer than any other decade since widespread thermometer measurements were introduced in the 1850s. Record heatwaves have occurred in Australia (January 2013), USA (July 2012), in Russia (summer 2010), and in Europe (summer 2003). The continuing effects of the warming climate are also seen in the increasing trends in ocean heat content and sea level, as well as in the continued melting of Arctic sea ice, glaciers and the Greenland ice sheet.