As the IPCC points out, confidence in the validity of this finding is based on the type, amount, quality, and consistency of evidence. The type of evidence used in the report were data collected in the past century, mechanistic understanding of the earth’s ecology, climate theory, mathematical models built to forecast the climate, and expert judgment. Observations of the climate system are based on direct measurements and remote sensing from satellites. Global-scale observations from the instrumental era began in the mid-19th century predominately for temperature. More comprehensive data has been collected since 1950s.
In their report the IPCC Working Group cites the overwhelming scientific consensus about the causes and results of climate change, but also calls for further assessments and projections, especially at regional level and in the oceans. Multiple lines of evidence confirm to the authors that the extra heat being trapped by greenhouse gases is warming the Earth’s atmosphere, surface, heating and acidifying the oceans, raising sea levels, and melting ice caps and glaciers. The report states the decade 2001-2010 was the warmest on record, a decade in which more temperature records were broken than any previous decade. However, the average surface temperature has actually not increased in the past decade.
Analysis of simulated natural climate variability by the MOHC using the climate models indicates that even with a long term warming rate of 0.3°F per decade, at least two periods without apparent temperature increase lasting a decade would be expected each century. The current pause in global surface temperature rise is not viewed as exceptional, based on those model simulations. The scientists cite two potential mechanisms to explain the recent pause; the first involves changes to the total energy received by the planet (radiative forcing), and the second involves the low frequency variability of the oceans and the way in which the oceans take up heat and store it below the surface in the deeper ocean. It is possible that a pause in surface warming could result from both mechanisms acting together though according to the MOHC radiative forcing by greenhouse gases has continued unabated; that heat is being held in the system but has not manifest as a rise in global mean surface temperature.
Observations of ocean heat content and of sea-level rise suggest that the additional heat has rather been absorbed in the oceans. Changes in the exchange of heat between the upper and deep ocean appear to have caused at least part of the pause in surface warming especially in the Pacific Ocean. In addition, the scientists point out the relative cooling influence of La Niña weather events. Since 2000 there have been no major El Niño events and indeed the tropical Pacific has been predominantly in La Niña states. These factors were not predicted in previous versions of the climate forecasting models. Thus, the IPCC cites the need for more research especially in the areas of deep sea temperatures.
IPCC and the scientific consensus have pivoted slightly in their focus to the oceans and sea level rise. The IPCC reports a Working Group consensus that ocean warming dominates the increase in energy stored in the earth’s climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010. Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise the rest is assembly attributed to the rise in sea levels that has been occurring unabated for over 10,000 years. “As the ocean warms, and glaciers and ice sheets reduce, global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years,” said Co-Chair Qin Dahe in the IPCC’s press release.
The IPCC Working Group expects global surface temperatures for the end of the 21st century to likely increase 2.7°F to 3.6°F relative to 1850 to 1900 time period. “Heat waves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions,” said Co-Chair Thomas Stocker in the press release. Dr. Stocker concluded his comments by reminding us that as a result of our past, present and expected future emissions of CO2, climate change is inevitable, and will persist for many centuries even if emissions of CO2 were to stop today.