There is an enormous amount of evidence, and agreement amongst scientists working in relevant fields, that the Earth's Climate is getting warmer due to human activities.
The Intergovernmental Panel on Climate Change is a body comprising experts in various relevant fields who periodically (re-)assess the evidence for climate change and its effects and for strategies to mitigate and adapt to its effects. The IPCC's working groups present their findings in individual reports and also produce a Synthesis Report, bringing together the Working Groups' report. The latest is 2014's Fifth Assessment Report. These findings are presented together via a new, beta, Climate Change 2014 Synthesis Report website (which also links to the separate reports).
- 1 Climate change, AGW, and global heating
- 2 Evidence and explanations
- 2.1 Skeptical Science
- 2.2 The Logic of Science
- 2.3 Peter Hadfield / potholer54
- 2.4 CO2 levels
- 2.5 Temperatures
- 2.6 Consensus
- 2.7 Statements by Scientific bodies
Climate change, AGW, and global heating
Climate change is also known as Anthropogenic Global Warming (AGW) or just "global warming" - and as "global heating". The term "heating" is increasingly being used instead of "warming" to indicate the severity of the phenomenon. The term "climate change" more accurately reflects that although the average temperature of the Earth is increasing, not all parts of the world are becoming hotter all the time (for example a polar vortex can bring unusually cold weather to usually temperate zones), and also that global heating results in other changes of climate such as rainfall and storm patterns, as well as temperature effects.
The term "anthropogenic" in AGW reflects the predominant role of human activities, such as unmitigated fossil fuel combustion, and changes in land use, on global heating.
Evidence and explanations
What's really warming the world? Bloomberg - graphic illustration of how observed temperatures match with models of changes due to earth's orbit, solar variation, volcanic activity, deforestation, ozone pollution, aerosol pollution, greenhouse gases
Climate Lab Book Ed Hawkins' blog
Berkeley Earth Global Mean Surface Temperature, 1850-2017 2 minute video showing global temperature changes
The 1981 TV documentary that warned about global warming Leo Hickman; Carbon Brief; 02 May 2017
- On the evening of Tuesday, 8 December, 1981, the UK’s only commercial TV channel, ITV, broadcast an hour-long documentary called “Warming Warning”. It was among the earliest occasions – possibly the earliest – anywhere in the world where a major broadcaster aired a documentary dedicated solely to the topic of human-caused climate change.
- The clips provide a poignant, historical insight into what scientists knew about climate change almost four decades ago – and how the world was beginning to react in terms of the resulting geopolitical, technological and societal ramifications. Many of themes still resonate strongly today.
- To put it in context, the documentary was broadcast seven years before Dr James Hansen’s famous “it is already happening now” Senate testimony in 1988, nine years before the first Intergovernmental Panel on Climate Change (IPCC) assessment report was published, and 25 years before Al Gore’s An Inconvenient Truth was released.
- Here is a summary of global warming and climate change myths, sorted by recent popularity vs what science says. Click the response for a more detailed response. You can also view them sorted by taxonomy, by popularity, in a print-friendly version, with short URLs or with fixed numbers you can use for permanent references.
The Logic of Science
- Claims that climate change isn’t happening:
- #1: It snowed, so global warming must not be true
- #2: The ice in Antarctica is actually increasing
- #3: Global warming has paused
- #4: Global warming wasn’t happening so they changed to name to climate change
- #5: The models have all been wrong
- #6: Polar bear numbers are actually increasing!
- Climate change isn’t caused by us
- #7: The climate has changed in the past, so the current warming is natural. It’s the sun, volcanoes, Milankovitch cycles, etc.
- #8: During past climate changes, the CO2 follows the temperature increase
- #9: CO2 only makes a small portion of the atmosphere
- #10: We only emit a tiny portion of the earth’s CO2
- #11: Water vapor is a far more potent greenhouse gas than CO2
- Scientists have been wrong before and/or they are incompetent and corrupt
- #12: In the 70’s scientists predicted an ice age
- #13: It’s just a theory, not a fact
- #14: But scientists have been wrong in the past, and we can’t be totally certain that climate change is true
- #15: There are thousands of scientists who disagree (e.g., the Oregon Petition)
- #16: “Climategate” showed that scientists are corrupt
- #17: Scientists are manipulating the data to make it look like warming!
- #18: It’s a liberal conspiracy/It’s all about the money!
- #19: But politicians and the media…
- #20: Climate change is being caused by the ozone hole (or vice versa)
- #21: But CO2 is actually good for plants
- #22: It’s not really a big problem, because the planet will only warm by a few degrees
- #23: It will make humans go extinct/it will be the end of the world
- #24: God is in control
- #25: Man is not powerful enough to cause climate change
Peter Hadfield / potholer54
Peter Hadfield has made a series of videos explaining the evidence for man-made global warming, and examining scientific and non-scientific arguments against it:
- Climate Change explained, and the myths debunked (all videos)
- There is a lot of inaccurate nonsense about climate science written in blogs and the media, whether exaggerating the effects of climate change or seeking to undermine the science behind it. This series checks the sources of these claims and shows how they have been misinterpreted or deliberately altered. I have no expertise in climatology, I am a former science journalist, so checking facts is what I do. And I always cite these sources so you can check them for yourselves. Along the way, I explain the real science as relayed by researchers in published papers, in a way that makes it easy to understand.
- A basic look at how climate scientists infer that man-made carbon gases are changing the climate, and how this view is contradicted by other climate scientists who are skeptics.
- This video... looks at alternative hypotheses explaining global warming. I am only looking at alternative hypotheses put forward by real, professional climate researchers, and the findings of real, professional climate researchers who disagree with them.
- I had planned to put several myths in this video, but discovered such an appalling web of deceit and fabrication in this first one that I felt I had no choice but to thoroughly debunk it.
- This video, the fourth in my Climate Change series, looks at urban myths spawned by two iconic films -- An Inconvenient Truth and The Great Global Warming Swindle. Whatever you "believe" about climate change, there is no excuse for the kind of exaggerations, fallacies and fabrications we see in films like these.
- More urban myths about climate change are busted as I look at the Earth's climate over the last 500 million years. What causes it to change? Since carbon dioxide was much higher in the past, why do climatologists say higher CO2 now poses a problem? And of course there's the familiar myth that CO2 can't influence temperatures because the climate was much colder in the past when carbon dioxide levels were much higher.
- ... a more sober analysis of those e-mails and what they mean. ... I've taken the two ... Phil Jones's e-mail about "Mike's Nature trick" and Kevin Trenberth's e-mail about a "travesty."
- Are climatologists censoring scientific journals and silencing alternative hypotheses on climate change?
- This video also looks at whether other planets are also warming, and an Internet myth that NASA is now attributing warming to the sun.
- a quote from Professor Phil Jones that there has been no global warming since 1995. But is that what he actually said? Once again, we need to go to the source -- Jones's own words -- rather than Internet gossip based on an interpretation of what he said.
- In 2005 the media told us we were on the brink of another ice age. What happened?
- Three more myths, misunderstood by both proponents and critics of climate science: Global Warming means more hurricanes, drowned islands and dead coral reefs. It's not that simple.
- a paper by researchers at NASA's Goddard Institute for Space Studies led to sensational headlines that the Earth will only warm by as much as 1.64 degrees centigrade -- in a couple of centuries. Sound too good to be true? Of course it does.
- This addresses a response to my video "Climate Change -- Hurricanes, atolls and corals," which, on investigation, revealed a major error by a news agency and a TV network. The moral of the story is that sticking the label "Global Warming" onto anything that moves is not going to help public understanding of climate science.
- 14. BP oil spills and an end to snow
- Monckton Bunkum Part 1 - Global cooling and melting ice
- Monckton Bunkum Part 2 - Sensitivity
- Monckton Bunkum Part 3 - Correlations and Himalayan glaciers
- Monckton Bunkum Part 4 -- Quotes and misquotes
- Monckton bunkum Part 5 -- What, MORE errors, my lord?
- This will have to be the last video in the Monckton Bunkum series, because he's made so many mistakes in his presentations it will take at least three more videos to debunk them all, and I'm getting tired of having to correct him.
- 20. Are cosmic rays causing global warming?
- 21. "Earth facing mini-ice age!!" say the media. Now for the science....
- 22. Climategate mark 2 -- the quotes and the context
- 23. Medieval Warm Period -- fact vs. fiction
- This video looks at the scientific research to answer three basic questions: 1) Was the Medieval Warm Period global? 2) Was it warmer than today? 3) And what does this all mean anyway?
- 24. Global warming has stopped? Again??
- 25. Climate Change -- The "800-year lag" unravelled
- 26. Science vs. the Feelies
- 27. The evidence for climate change WITHOUT computer models or the IPCC (sources)
- 28. The consequences of climate change (in our lifetimes) continued
- Why global temperatures never go up in straight lines
The Last Time CO2 Was This High, Humans Didn’t Exist Andrew Freedman; Climate Central; 3 May 2013
- The last time there was this much carbon dioxide (CO2) in the Earth's atmosphere, modern humans didn't exist. Megatoothed sharks prowled the oceans, the world's seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now. As we near the record for the highest CO2 concentration in human history — 400 parts per million — climate scientists worry about where we were then, and where we're rapidly headed now. According to data gathered at the Mauna Loa Observatory in Hawaii, the 400 ppm mark may briefly be exceeded this month, when CO2 typically hits a seasonal peak in the Northern Hemisphere, although it is more likely to take a couple more years until it stays above that threshold, according to Ralph Keeling, a researcher at the Scripps Institute of Oceanography.
Sea surface temperature measurements
Possible artifacts of data biases in the recent global surface warming hiatus Thomas R. Karl, Anthony Arguez, Boyin Huang, Jay H. Lawrimore, James R. McMahon, Matthew J. Menne, Thomas C. Peterson, Russell S. Vose, Huai-Min Zhang; AAAS Science; 26 Jun 2015
- Previous analyses of global temperature trends during the first decade of the 21st century seemed to indicate that warming had stalled. This allowed critics of the idea of global warming to claim that concern about climate change was misplaced. Karl et al. now show that temperatures did not plateau as thought and that the supposed warming “hiatus” is just an artifact of earlier analyses. Warming has continued at a pace similar to that of the last half of the 20th century, and the slowdown was just an illusion.
- Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here, we present an updated global surface temperature analysis that reveals that global trends are higher than those reported by the Intergovernmental Panel on Climate Change, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.
Hausfather analysis of NOAA data
NOAA challenged the global warming ‘pause.’ Now new research says the agency was right. Chris Mooney; Washington Post; 4 Jan 2017
- In the summer of 2015, a team of federal scientists at the National Oceanic and Atmospheric Administration published a blockbuster paper in Science that appeared to wipe away one of global warming doubters’ favorite arguments. The skeptics had for years suggested that following the then-record warm year of 1998 and throughout the beginning of the 21st century, global warming had slowed down or “paused.” But the 2015 paper, led by NOAA’s Thomas Karl, employed an update to the agency’s influential temperature dataset, and in particular to its record of the planet’s ocean temperatures, to suggest that really, the recent period was perfectly consistent with the much longer warming trend.
- This didn’t merely surprise some scientists (who had been busily studying why global warming had appeared to moderate its rate somewhat in the early 21st century). It actually led to a congressional subpoena from Rep. Lamar Smith, chair of the House Committee on Science, who charged that “NOAA’s decision to readjust historical temperature records has broad national implications” and requested more information on why NOAA had made the dataset adjustment, including data and communications from the scientists involved.
- That controversy is likely to be stirred anew in the wake of a new study, published Wednesday in Science Advances, that finds the NOAA scientists did the right thing in adjusting their dataset. In particular, the new research suggests that the NOAA scientists correctly adjusted their record of ocean temperatures in light of known biases in some observing systems — and indeed, that keepers of other top global temperature datasets should do likewise.
New analysis shows Lamar Smith’s accusations on climate data are wrong SCOTT K. JOHNSON; Ars Technical; 1 Jan 2017
- It wasn't a political plot—temperatures really did get warmer.
- roducing a dataset of global surface temperature is more complicated than calculating the average of a few numbers—especially when it comes to the temperature of the seawater that covers most of the planet’s surface. Since the oceans are sparsely monitored, researchers need all the measurements they can get their hands on, which include instruments on buoys and measurements made in a variety of ways on ships.
- Not only do different measurement techniques have the potential to give slightly different temperature readings, but practices change over time. At one point, wooden buckets were thrown over the side and hauled up. Then it was canvas buckets, which allowed a little more evaporative cooling on the way up. Then it was mainly sensors at the engine’s coolant water intake. And in recent years, the number of scientific buoys and autonomous floats has exploded. Getting all the data collected over the years linked up on an apples-to-apples basis takes careful analysis.
Debunking the myth of climate change 'hiatus': Where did it come from? Eva Botkin-Kowacki; CS Monitor; 4 Jan 2017
- A team of researchers independently replicated NOAA's recalibration of sea surface temperature data to investigate what really happened from 1998 to 2012.
- From around 1998 to 2012, the rise in global temperatures seemed to plateau, according to NOAA's Extended Reconstruction Sea Surface Temperature (ERSST) dataset. To most climate scientists, this so-called hiatus was another puzzle of our complex climate system for them to work out. But to people who were already skeptical of global warming, this data was evidence for the idea that human-induced climate change is a hoax. But the data itself was unsound, scientists now say. There is no evidence of a hiatus.
- Climate scientists use sea surface temperature data in their calculations of global warming trends. But "a fair bit of the apparent hiatus seems to be due to problems in our ocean measurements, and not a real thing," as study lead author Zeke Hausfather, an energy systems analyst and data scientist at the University of California Berkeley and Berkeley Earth, tells The Christian Science Monitor in a phone interview.
- Dr. Hausfather and his colleagues aren't the first team to say this. But in 2015, when National Oceanic and Atmospheric Administration (NOAA) scientists first identified the errors in the data, political turmoil ensued. So, as replication is a tenet of science, Hausfather and his colleagues set about seeing if they could come to the same conclusion as the NOAA team. Their results are detailed in a paper published Wednesday in the journal Science Advances.
Assessing recent warming using instrumentally homogeneous sea surface temperature records Zeke Hausfather, Kevin Cowtan, David C. Clarke, Peter Jacobs, Mark Richardson, Robert Rohde; AAAS Science Advances; 4 Jan 2017
- Sea surface temperature (SST) records are subject to potential biases due to changing instrumentation and measurement practices. Significant differences exist between commonly used composite SST reconstructions from the National Oceanic and Atmospheric Administration’s Extended Reconstruction Sea Surface Temperature (ERSST), the Hadley Centre SST data set (HadSST3), and the Japanese Meteorological Agency’s Centennial Observation-Based Estimates of SSTs (COBE-SST) from 2003 to the present. The update from ERSST version 3b to version 4 resulted in an increase in the operational SST trend estimate during the last 19 years from 0.07° to 0.12°C per decade, indicating a higher rate of warming in recent years. We show that ERSST version 4 trends generally agree with largely independent, near-global, and instrumentally homogeneous SST measurements from floating buoys, Argo floats, and radiometer-based satellite measurements that have been developed and deployed during the past two decades. We find a large cooling bias in ERSST version 3b and smaller but significant cooling biases in HadSST3 and COBE-SST from 2003 to the present, with respect to most series examined. These results suggest that reported rates of SST warming in recent years have been underestimated in these three data sets.
- Multiple studies published in peer-reviewed scientific journals1 show that 97 percent or more of actively publishing climate scientists agree: Climate-warming trends over the past century are extremely likely due to human activities. In addition, most of the leading scientific organizations worldwide have issued public statements endorsing this position. The following is a partial list of these organizations, along with links to their published statements and a selection of related resources.
CLIMATE SCIENCE SURVEY - Questions and Responses Bart Strengers, Bart Verheggen, Kees Vringer; PBL Netherlands Environmental Assessment Agency; 10 Apr 2015
- In the Spring of 2012, PBL, in collaboration with other researchers from the Netherlands and Australia, conducted a detailed survey about climate science. More than 1800 international scientists studying various aspects of climate change, including e.g. climate physics, climate impacts and mitigation, responded to the questionnaire. Certain results were selected from this survey, namely those pertaining to the causes of recent global warming (attribution), and have since been published in Environmental Science and Technology (ES&T).
- This document presents the responses to each survey question, both as an absolute number of responses and as a fraction of the total. In some cases, the responses were also divided into seven groups of respondents: co-authors of the Working Group I report of IPCC AR4 (‘AR4 authors’); signatories of public declarations critical of mainstream climate science as embodied by IPCC (‘unconvinced’); and four subgroups divided by their self-declared number of climate-related articles published in peer-reviewed scientific journals (0–3; 4–10; 11–30; more than 30). The four subgroups constitute similar numbers of respondents.
- Consensus is more like 99.9% than 97%
Essentially All Climate Scientists Agree: Man-Made Global Warming is Real Henry Auer; The Energy Collective; 4 May 2016
- Virtually Complete Unanimity of Acceptance of Man-Made Global Warming. James Lawrence Powell recently published an article (Bulletin of Science, Technology & Society 1–4, 2016; DOI:10.1177/0270467616634958) which found that during 2013 and 2014 only 0.0058% of authors of peer-reviewed journal articles rejected the reality of man-made global warming. Of the almost 70,000 authors of those articles only 4 reached that conclusion, giving a ratio of 1:17,352.
Climate Scientists Virtually Unanimous: Anthropogenic Global Warming Is True James Lawrence Powell; Bulletin of Science, Technology and Society; 2016
- The extent of the consensus among scientists on anthropogenic global warming (AGW) has the potential to influence public opinion and the attitude of political leaders and thus matters greatly to society. The history of science demonstrates that if we wish to judge the level of a scientific consensus and whether the consensus position is likely to be correct, the only reliable source is the peer-reviewed literature. During 2013 and 2014, only 4 of 69,406 authors of peer-reviewed articles on global warming, 0.0058% or 1 in 17,352, rejected AGW. Thus, the consensus on AGW among publishing scientists is above 99.99%, verging on unanimity. The U.S. House of Representatives holds 40 times as many global warming rejecters as are found among the authors of scientific articles. The peer-reviewed literature contains no convincing evidence against AGW.
Benestad et al
Learning from mistakes in climate research Rasmus E. Benestad, Dana Nuccitelli, Stephan Lewandowsky, Katharine Hayhoe, Hans Olav Hygen, Rob van Dorland, John Cook; Theoretical and Applied Climatology; Nov 2016
- Among papers stating a position on anthropogenic global warming (AGW), 97 % endorse AGW. What is happening with the 2 % of papers that reject AGW? We examine a selection of papers rejecting AGW. An analytical tool has been developed to replicate and test the results and methods used in these studies; our replication reveals a number of methodological flaws, and a pattern of common mistakes emerges that is not visible when looking at single isolated cases. Thus, real-life scientific disputes in some cases can be resolved, and we can learn from mistakes. A common denominator seems to be missing contextual information or ignoring information that does not fit the conclusions, be it other relevant work or related geophysical data. In many cases, shortcomings are due to insufficient model evaluation, leading to results that are not universally valid but rather are an artifact of a particular experimental setup. Other typical weaknesses include false dichotomies, inappropriate statistical methods, or basing conclusions on misconceived or incomplete physics. We also argue that science is never settled and that both mainstream and contrarian papers must be subject to sustained scrutiny. The merit of replication is highlighted and we discuss how the quality of the scientific literature may benefit from replication.
Those 3% of scientific papers that deny climate change? A review found them all flawed Katherine Ellen Foley; Quartz; 5 Sep 2017
- It’s often said that of all the published scientific research on climate change, 97% of the papers conclude that global warming is real, problematic for the planet, and has been exacerbated by human activity.
- But what about those 3% of papers that reach contrary conclusions? Some skeptics have suggested that the authors of studies indicating that climate change is not real, not harmful, or not man-made are bravely standing up for the truth, like maverick thinkers of the past. (Galileo is often invoked, though his fellow scientists mostly agreed with his conclusions—it was church leaders who tried to suppress them.)
- Not so, according to a review published in the journal of Theoretical and Applied Climatology. The researchers tried to replicate the results of those 3% of papers—a common way to test scientific studies—and found biased, faulty results.
Baldwin and Lammers / University of Cologne
Past-focused environmental comparisons promote proenvironmental outcomes for conservatives Matthew Baldwina, Joris Lammers; PNAS; 31 Oct 2016
- Political polarization on important issues can have dire consequences for society, and divisions regarding the issue of climate change could be particularly catastrophic. Building on research in social cognition and psychology, we show that temporal comparison processes largely explain the political gap in respondents’ attitudes towards and behaviors regarding climate change. We found that conservatives’ proenvironmental attitudes and behaviors improved consistently and drastically when we presented messages that compared the environment today with that of the past. This research shows how ideological differences can arise from basic psychological processes, demonstrates how such differences can be overcome by framing a message consistent with these basic processes, and provides a way to market the science behind climate change more effectively.
- Conservatives appear more skeptical about climate change and global warming and less willing to act against it than liberals. We propose that this unwillingness could result from fundamental differences in conservatives’ and liberals’ temporal focus. Conservatives tend to focus more on the past than do liberals. Across six studies, we rely on this notion to demonstrate that conservatives are positively affected by past- but not by future-focused environmental comparisons. Past comparisons largely eliminated the political divide that separated liberal and conservative respondents’ attitudes toward and behavior regarding climate change, so that across these studies conservatives and liberals were nearly equally likely to fight climate change. This research demonstrates how psychological processes, such as temporal comparison, underlie the prevalent ideological gap in addressing climate change. It opens up a promising avenue to convince conservatives effectively of the need to address climate change and global warming.
Environmental messages that promote a return to a positive past found to be more effective in convincing conservatives Bob Yirka; Phys.org; 13 Dec 2016
- A pair of researchers with the University of Cologne in Germany has found that phrasing pro-environmental messages in past-focused ways was received more warmly by people who described themselves as conservatives than messages that warned of future problems. In their paper published in Proceedings of the National Academy of Sciences, Matthew Baldwin and Joris Lammers describe the study they carried out with online volunteers and why they believe their results could have a real-world impact.
Wouldn’t it be great if the planet went back to how it used to be? CATHLEEN O'GRADY; Ars Technica; 16 Dec 2016
- Climate skepticism decreases if the message is past-oriented.
Statements by Scientific bodies
The Geological Society of London
- Climate change is a defining issue for our time. The geological record contains abundant evidence of the ways in which Earth’s climate has changed in the past. That evidence is highly relevant to understanding how it may change in the future.
- The Council of the Society is issuing this statement as part of the Society’s work “to promote all forms of education, awareness and understanding of the Earth and their practical applications for the benefit of the public globally”. The statement is intended for non-specialists and Fellows of the Society. It is based on analysis of geological evidence, and not on analysis of recent temperature or satellite data, or climate model projections. It contains references to support key statements, indicated by superscript numbers, and a reading list for those who wish to explore the subject further.
- The Geological Society published a Statement on 'Climate Change: Evidence from the Geological Record' in November 2010. In light of further research that has been published since then, the Geological Society reconvened the expert working group that drafted the 2010 Climate Change Statement to consider whether it was still fit for purpose, and if necessary to amend or add to it.
- The working group and Council have concluded that the 2010 Climate Change Statement continues to be valid, and does not need to be amended. Instead, the working group has produced an addendum setting out new research findings relevant to the questions raised in the original statement.
- A non-technical summary of the key points from the addendum is set out below, aimed principally at non-specialists and Fellows of the Society with a general interest. This is followed by the full technical version of the addendum, for those who wish to read in more detail about advances in the relevant research. The full technical version includes references to the published papers on which it draws. It is intended to be read alongside the original 2010 Climate Change Statement, and follows the same Q&A format.
- Since our original 2010 statement, new climate data from the geological record have arisen which strengthen the statement’s original conclusion that CO2 is a major modifier of the climate system, and that human activities are responsible for recent warming.
- Palaeoclimate records are now being used widely to test the validity of computer climate models used to predict climate change. Palaeoclimate models can simulate the large-scale gradients of past change, but tend not to accurately reproduce fine-scale spatial patterns. They also have a tendency to underestimate the magnitude of past changes. Nevertheless they are proving to be increasingly useful tools to aid thinking about the nature and extent of past change, by providing a global picture where palaeoclimate data are geographically limited.
- Geologists have recently contributed to improved estimates of climate sensitivity (defined as the increase in global mean temperature resulting from a doubling in atmospheric CO2 levels). Studies of the Last Glacial
- Maximum (about 20,000 years ago) suggest that the climate sensitivity, based on rapidly acting factors like snow melt, ice melt and the behaviour of clouds and water vapour, lies in the range 1.5°C to 6.4°C. Recent research has given rise to the concept of ‘Earth System sensitivity’, which also takes account of slow acting factors like the decay of large ice sheets and the operation of the full carbon cycle, to estimate the full sensitivity of the Earth System to a doubling of CO2. It is estimated that this could be double the climate sensitivity.