Carbon Trust summary of IPCC's 5th Assessment Report

IPCC Summary for Policymakers – AR5 2013

The Intergovernmental Panel on Climate Change is an international body created by the UN in 1988 to collect and synthetize the latest science on climate change. The IPCC publishes a summary report every 5 years presenting all the gathered evidence. The report is endorsed by all UN member States.

The Fifth Assessment Report (AR5), published in September 2013, considered new scientific evidence, based on many independent scientific analyses from observations of the climate system, historical climate archives, theoretical studies of climate processes and simulations using climate models, to conclude that there is a 95% probability that human action is the dominant cause of climate change. This represents an increase in certainty from 90% in the previous report. Natural variables such as total solar irradiance have not been found to represent significant contributions to the warming trend.

The report points out that the warming of the climate system since 1850 is unequivocal and unprecedented over a period of millennia, as confirmed by historical climate reconstructions. The 30 year period between 1983-2012 is likely to have been the warmest in the past 1,400 years. Overall, global combined land and ocean surface temperatures have increased by 0.85˚C from 1850 to 2012.

There is very high confidence that climate models can reproduce these patterns on a continental-scale, and accurately represent temperature changes as a result of anthropogenic (man-made) forcings. However there are differences between simulated and observed trends over periods as short as 10 to 15 years.

When considering shorter timeframes, the start and end-year can have large effects on the trend due to short-term natural variability, and do not reflect long-term climate trends. For example, the 15 years from 1998, a strong El Nino year, to 2012, have a mean global surface warming trend of 0.05˚C per decade, but shifting the period just two years to 1996-2010 increases the warming trend to 0.13˚C per decade, in line with the 1951-2012 decadal trend of 0.12˚C. Models are not expected to reproduce the timing of internal natural climate variability over such short timescales.

A similar rate of warming has been observed in the upper levels of the ocean (up to 75m in depth) over the period 1971-2010, with increased confidence in the measurements since the previous IPCC report. While there might have been a slowdown in the rate of increase of ocean heat content in the first 700m between 1993-2010, the heat uptake continued to increase unabated in the deeper ocean, 700m to 2000m, from 1993 to 2009.This could partially explain the slow down in global surface temperature increase, as heat is transferred to the deep ocean.

The atmospheric warming has resulted in the increase of extreme weather and climate events in various areas of the globe. The frequency of heat waves has increased in large parts of Europe, Asia and Australia, while the frequency or intensity of heavy precipitation events has increased in North America and Europe.

Changes in the icy areas of the world have been far more substantial. The average rate of ice loss from glaciers worldwide increased by 18% in the last 20 years when compared to the overall 1971 – 2009 trend, to 275Gt per year; ice loss from the Greenland ice sheet more than sextupled, from 34 Gt yr 1992-2001 to 215 Gt yr 2002-2011; and ice loss from the Antarctic ice sheet quintupled from 30 Gt yr 1992-2001 to 147 Gt yr 2002-2011. Summer arctic sea ice extent declined by up to 13.6% per decade in the period 1979-2012, a retreat unprecedented in the past 1,450 years.

As a result of this, since the mid 19th century sea level rise has been larger than the mean rate during the previous two millennia. 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.

COconcentrations have increased by 40% since pre-industrial times, reaching levels that are above those of the past 800,000 years. In addition to the warming, this has caused ocean acidification

If current emission trends continue, warming is likely to exceed two degrees, and could possibly exceed 4˚C by 2100. This will result in large changes to most natural cycles. Over the next 100 years precipitation is likely to increase in wet regions and decrease in dry regions, exacerbating floods and droughts. The ocean will continue to warm and expand, raising sea levels and weakening the Gulf Stream by up to 34% in the highest emission scenario, RP8.5. A complete collapse of the Gulf Stream beyond the 21st century cannot be excluded, which would radically affect the climate of the Northern hemisphere, chiefly Europe and North America.

Most aspects of climate change will persist for many centuries even if CO2 emissions are stopped, with between 15% and 40% of anthropogenic CO2 remaining in the atmosphere longer than 1,000 years.

Overall, the report represents a confirmation of global warming trends and the expected negative impacts of current emission pathways on the global climate. Scientists are now all but certain that climate change is mostly caused by human action, and that it is already leading to changes in regional weather patterns, with extreme events on the increase.

View the IPCC Fifth Assessment Report 'Climate Change 2013: The Physical Science Basis'.

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