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FEATURE: At-a-glance guide to the IPCC’s Fifth Assessment Report findings on physical science

Dr Qamar Chaudhry, an eminent climate scientist and Deputy Director of CDKN’s Asia programme, selects some of the headlines from the Intergovernmental panel on Climate Change (IPCC)’s recently released report on the physical science of climate change. He highlights some key ways in which scientific understanding has evolved since the IPCC’s last Assessment Report in 2007.

The IPCC is the most authoritative, intergovernmental scientific body on climate change under the auspices of the United Nations (UN). It reviews and assesses the most recent scientific, technical and socio-economic information produced, and form a clear scientific view on the current state of knowledge in climate change and its potential  impacts. So far, the IPCC has produced four assessment reports. Thousands of scientists and experts worldwide voluntarily contribute to the preparation of these IPCC reports.  Following are some salient findings of the first section of the IPCC’s Fifth Assessment Report (AR5): that of Working Group I on the physical science.

Understanding the climate system and its recent changes

Evidence of the effects of human influence on the climate system has continued to accumulate and strengthen since the Fourth Assessment Report (AR4). The consistency of observed and modeled changes across the climate system, including regional temperatures, the water cycle, global energy budget, cryosphere and oceans (including aspects of ocean acidification), point to global climate change that results primarily from anthropogenic increases in greenhouse gas concentrations.

Energy budget and heat content: Earth has been in radiative imbalance, with more energy from the sun entering than exiting the top of the  atmosphere, since at least 1970. It is virtually certain that Earth has gained substantial energy from 1971–2010.

Surface temperatures: Globally-averaged land and ocean surface temperatures have increased since the beginning of the 20th century and this  warming is virtually certain. This  increase is about 0.8°C over the period 1901–2010 and about 0.5°C over the period 1979–2010. Several advances since the last IPCC report (AR4) have increased scientific confidence in the attribution of these warming  trends to human influence.  The report  has acknowledged  that the rate of global warming has slowed since 1998, with the average surface global temperature staying steady in these first years of the 21st century. The rate of warming seen between 1998 and 2012 is flatter than the trend seen since 1951.

Water cycle : New observations and their combination with climate model simulations now permit the attribution of some changes in the water cycle since 1950 to anthropogenic influences. Global, land-based precipitation shows little change since 1900,  although the confidence is  low prior to 1950 and medium afterwards. This is different from AR4, which reported that global precipitation averaged over land areas has increased, with most of the increase occurring in the early to mid 20th century.

It is virtually certain that the projected increase in global-scale precipitation in the 21st century will be much smaller, but it would not be uniform ,with some regions experiencing increased rainfall, and others facing a decrease.  Many mid-latitude arid and semi-arid regions will likely experience less precipitation and many moist, mid-latitude regions will likely experience more precipitation.

Runoff: The most recent and most comprehensive analyses of river runoff, which include newly assembled  observational records, do not support the AR4 conclusion that global runoff increased during the 20th century. Average runoff has not changed in the majority of rivers, but year-to-year variability has increased.

Glaciers and ice sheets: The time series of measured changes in glacier length and area, as well as estimates of volume and mass change give robust evidence in high agreement that globally, glaciers continue to shrink and lose mass. There are, however, notable regional exceptions to these trends.

Sea level rise: It is unequivocal that global mean sea level is rising. It is virtually certain that it has risen  between 1.4 to 2.0 mm per year during  the 20th century, and between 2.7 and 3.7 mm per year since 1993. Ocean thermal expansion and glacier mass loss are very likely the dominant contributors to sea level rise during the 20th century.

Extreme events: Recent analyses of extreme events generally support the AR4 and SREX conclusions, according to which it is very likely that the  overall number of cold days and nights has decreased and the overall number of warm days and nights has  increased on the global scale between 1951 and 2010.

Consistent with AR4 conclusions, it is likely that the number of heavy precipitation  events has increased in more regions than it has decreased since 1950.

Ocean acidification: There is very high confidence that oceanic uptake of anthropogenic CO2 has resulted in gradual acidification of seawater and decreasing pH.

Climate Stabilisation and irreversibility: Many aspects of climate change will persist for centuries even if concentrations of greenhouse gases are stabilised. This represents a substantial multi-century commitment created by human activities today.

Projections and observations comparison: Results of projected changes in CO2, global mean surface temperature and global mean sea level from previous four IPCC assessment reports are quantitatively compared with the observational  estimates. The comparison highlights the evolution in understanding of how the climate system responds to changes in both natural and anthropogenic forcing and provides an assessment of how the projections compare with observational estimates.

CDKN is launching a new project to bring the findings of the IPCC’s Fifth Assessment Report to policy makers in developing regions. Read more about our new project – Outreach on the Fifth Assessment Report of the IPCC.

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