Human influence on the climate system is clear. This is evident in most regions of the globe, a new assessment by the Intergovernmental Panel on Climate Change (IPCC) concludes.
It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. The evidence for this has grown, thanks to more and better observations, an improved understanding of the climate system response and improved climate models.
Warming in the climate system is unequivocal and since 1950 many changes have been observed throughout the climate system that are unprecedented over decades to millennia. Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850, reports the Summary for Policymakers of the IPCC Working Group I assessment report, Climate Change 2013: the Physical Science Basis, approved on Friday by member governments of the IPCC in Stockholm, Sweden.
“Observations of changes in the climate system are based on multiple lines of independent evidence. Our assessment of the science finds that the atmosphere and ocean have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased,” said Qin Dahe, Co-Chair of IPCC Working Group I.
Thomas Stocker, the other Co-Chair of Working Group I said: “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”
“Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5°C relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2°C for the two high scenarios,” said Co-Chair Thomas Stocker. “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,” he added. Projections of climate change are based on a new set of four scenarios of future greenhouse gas concentrations and aerosols, spanning a wide range of possible futures. The Working Group I report assessed global and regional-scale climate change for the early, mid-, and later 21st century.
“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. The report finds with high confidence that ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010.
Co-Chair Thomas Stocker concluded: “As a result of our past, present and expected future emissions of CO2, we are committed to climate change, and effects will persist for many centuries even if emissions of CO2 stop.”
Rajendra Pachauri, Chair of the IPCC, said: “This Working Group I Summary for Policymakers provides important insights into the scientific basis of climate change. It provides a firm foundation for considerations of the impacts of climate change on human and natural systems and ways to meet the challenge of climate change.” These are among the aspects assessed in the contributions of Working Group II and Working Group III to be released in March and April 2014. The IPCC Fifth Assessment Report cycle concludes with the publication of its Synthesis Report in October 2014.
The IPCC supplies policy-relevant information about climate change to the world’s governments. Its Fifth Assessment Report (AR5) will be considered by negotiators responsible for concluding a new agreement under the United Nations Framework Convention on Climate Change (UNFCCC) in 2015.
Some of the report’s key findings include:
The global mean average surface temperature rose by 0.89°C from 1901 to 2012. Each of the last three decades has been warmer than all preceding decades since 1850. In the Northern Hemisphere, 1983-2012 was likely the warmest 30-year period of the last 1400 years. The first decade of the 21st century has been the warmest of all (WMO’s The Global Climate 2001-2010 estimates the global average surface temperature for that decade at 14.47°C). Global average temperatures will likely rise by another 0.3°C to 0.7°C in the period 2016-2035. Averaged over the period 2081-2100, the global surface temperature is likely to exceed pre-industrial levels by 1.5°C or even (depending on future greenhouse gas emissions) 2°C.
Changes in many extreme weather and climate events have been observed since about 1950. It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. In in large parts of Europe, Asia and Australia, it is likely that the frequency of heat waves has increased.
It is virtually certain that the upper ocean (down to 700m) has warmed from 1971 to 2010. The deep ocean below 3000m has also likely warmed since the 1990s, when sufficient observations became available. Ocean warming accounts for most of the change in the amount of incoming solar energy stored by the Earth, accounting for about 93% of it between 1971 and 2010. The global ocean will continue to warm during the 21st century. Heat will penetrate from t he surface to the deep ocean and affect ocean circulation.
The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. The global mean sea level rose by around 19 cm from 1901 to 2010 due to increased ocean warming and melting glaciers and ice sheets. The rate of rise accelerated between 1993 and 2010, and it is very likely to increase further during the 21st century and beyond. The report notes that, during the last interglacial, when the climate was 2°C warmer than pre-industrial levels, maximum global sea levels were 5 to 10 meters higher than they are today.
Seawater has become more acidic (its pH has decreased by 0.1) since the beginning of the industrial era due to humanity’s carbon dioxide emissions; it will continue to acidify during the 21st century.
It is very likely that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises Some scenarios foresee a nearly ice-free Arctic Ocean in September before mid-century.
There is very high confidence that glaciers have continued to shrink and lose mass world-wide, with very few exceptions. By 2100, glacial volume could, under one scenario, decline further by as much as 35-85%. Meanwhile, the extent of Northern Hemisphere snow cover has decreased since the mid-20th century, especially in spring, and this decline, too, will continue.
It is likely that human influences have affected the global water cycle and its patterns since 1960. For example, in recent decades precipitation has increased in the mid-latitude land areas of the Northern Hemisphere.
Three years in the making, the “Physical Science Basis” volume of the Fifth Assessment Report was produced by over 250 scientists. Additional volumes on impacts, mitigation and a synthesis will be released over the coming year. The IPCC does not conduct new research. Instead, its mandate is to make policy-relevant assessments of the existing worldwide literature on the scientific, technical and socio-economic aspects of climate change. Its reports have played a major role in inspiring governments to adopt and implement the United Nations Framework Convention on Climate Change.
See separate Fact Sheet of Headline Statements from the WGI AR5 Summary for Policymakers, available at http://www.climatechange2013.org.
Sources: IPCC and The UK Guardian