Although the summary tries to be as non-technical as possible, it is after all a summary of the science, so it incorporates some scientific terminology necessary to convey the facts. It also includes some numbers, such as "240" and "3.6".
The report attempts to clarify which aspects of climate change science are widely agreed, which others have achieved consensus but where further research is expected to give more clarity, and which are not yet well understood.
It begins with a dozen paragraphs of "some background science", explaining very broadly what the greenhouse effect is, what is meant by "climate forcing" and "climate change", and why what may seem like small forcings of a few Watts per square meter can create the profound climate changes seen over past millennia.
At the risk of offering a précis of a summary, here are some of the key points of the report.
Aspects of climate change on which there is wide agreement
- "Averaged over the globe, the surface has warmed by about 0.8°C (with an uncertainty of about ±0.2°C) since 1850."
- "Each decade since the 1970s has been clearly warmer (given known uncertainties) than the one immediately preceding it. The decade 2000-2009 was, globally, around 0.15°C warmer than the decade 1990-1999."
- Other changes include "increases in the average temperature of both the upper 700m of the ocean and the troposphere (the atmosphere up to 10-18km), widespread (though not universal) decreases in the length of mountain glaciers and increases in average sea level."
- "Global-average CO2 concentrations have been observed to increase from levels of around 280 parts per million (ppm) in the mid-19th century to around 388 ppm by the end of 2009."
- "Present-day concentrations are higher than any that have been observed in the past 800,000 years, when CO2 varied between about 180 and 300 ppm."
- "Various lines of evidence point strongly to human activity being the main reason for the recent increase, mainly due to the burning of fossil fuels (coal, oil, gas) with smaller contributions from land-use changes and cement manufacture."
- "About half of the CO2 emitted by human activity since the industrial revolution has remained in the atmosphere."
- "The concentration of methane has more than doubled in the past 150 years; this recent and rapid increase is unprecedented in the 800,000 year record and evidence strongly suggests that it arises mainly as a result of human activity."
- "These additional gases have caused a climate forcing during the industrial era of around 2.9 Wm-2 [Watt per square meter], with an uncertainty of about ±0.2 Wm-2."
- "The net effect of all human activity has caused a positive climate forcing of around 1.6 Wm-2 with an estimated uncertainty of about ±0.8 Wm-2."
- "Changes in CO2 can lead to climate change and climate change can also alter the concentrations of CO2."
Aspects of climate change where there is a wide consensus but continuing debate and discussion
- "Current understanding indicates that even if there was a complete cessation of emissions of CO2 today from human activity, it would take several millennia for CO2 concentrations to return to preindustrial concentrations."
- "Natural forcing due to sustained variations in the energy emitted by the Sun over the past 150 years is estimated to be small (about 0.12 Wm-2)" but this remains an active area of research.
- "Particles have caused a negative climate forcing of around 0.5 Wm-2 with an uncertainty of ±0.2 Wm-2."
- "Climate models indicate that the overall climate sensitivity (for a hypothetical doubling of CO2 in the atmosphere) is likely to lie in the range 2°C to 4.5°C," with this wide range due to "uncertainties in how much water vapour amounts will change, and how these changes will be distributed in the atmosphere, in response to a warming."
- "Unless [internal climate variability] has been grossly underestimated,
the observed climate change must result from natural and/or human-induced climate forcing."
- "When only natural climate forcings are put into climate models, the models are incapable of reproducing the size of the observed increase in global-average surface temperatures over the past 50 years. However, when the models include estimates of forcings resulting from human activity, they can reproduce the increase."
- "The observed vertical and latitudinal variations of temperature change are also broadly consistent with those expected from a dominant role for human activity. There is an ongoing controversy concerning whether or not the increased warming with height in the tropical regions given by climate models is supported by satellite measurements."
- "The IPCC’s best estimate was that globally averaged surface temperatures would be between 2.5 - 4.7°C higher by 2100 compared to pre-industrial levels. The full range of projected temperature increases by 2100 was found to be 1.8 - 7.1°C based on the various scenarios and uncertainties in climate sensitivity."
- "Climate models tend to predict that precipitation will generally increase in areas with already high amounts of precipitation and generally decrease in areas with low amounts of precipitation."
- "Because of the thermal expansion of the ocean, it is very likely that for many centuries the rate of global sea-level rise will be at least as large as the rate of 20 cm per century that has been observed over the past century."
Aspects that are not well understood
- "Projections of climate change are sensitive to the details of the representation of clouds in models. Particles originating from both human activities and natural sources have the potential to strongly influence the properties of clouds, with consequences for estimates of climate forcing. Current scientific understanding of this effect is poor." [Or, as Joni Mitchell wrote in 1967, "I've looked at clouds from both sides now, From up and down and still somehow, It's cloud's illusions I recall; I really don't know clouds at all."]
- "The future strength of the uptake of CO2 by the land and oceans (which together are currently responsible for taking up about half of the emissions from human activity…) is very poorly understood, particularly because of gaps in our understanding of the response of biological processes to changes in both CO2 concentrations and climate."
- "There is currently insufficient understanding of the enhanced melting and retreat of the ice sheets on Greenland and West Antarctica to predict exactly how much the rate of sea level rise will increase above that observed in the past century ... for a given temperature increase."
- "There is little confidence in specific projections of future regional climate change, except at continental scales."
The authors conclude
- "There is strong evidence that changes in greenhouse gas concentrations due to human activity are the dominant cause of the global warming that has taken place over the last half century. This warming trend is expected to continue as are changes in precipitation over the long term in many regions. Further and more rapid increases in sea level are likely which will have profound implications for coastal communities and ecosystems."
- "Like many important decisions, policy choices about climate change have to be made in the absence of perfect knowledge. Even if the remaining uncertainties were substantially resolved, the wide variety of interests, cultures and beliefs in society would make consensus about such choices difficult to achieve. However, the potential impacts of climate change are sufficiently serious that important decisions will need to be made. Climate science – including the substantial body of knowledge that is already well established, and the results of future research – is the essential basis for future climate projections and planning, and must be a vital component of public reasoning in this complex and challenging area."
The report is available in PDF here.
The Royal Society of London for the Improvement of Natural Knowledge, better known as just the "Royal Society", is one of the world's premiere national scientific organizations. It acts as the United Kingdom's "academy of sciences".
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