Landmark 2013 IPCC Report: 95% Chance Most of Global Warming is Human-Caused

Published: 10:50 AM GMT on September 27, 2013

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased." Thus opens the landmark 2013 Intergovernmental Panel on Climate Change (IPCC) report issued today. Working without pay, hundreds of our most dedicated and talented climate experts have collaborated over a six-year period to create the most comprehensive and authoritative scientific document on climate change ever crafted. The first 31 pages of what will be a 4,000-page tome was released this morning after an all-night approval session that stretched until 6:30 this morning in Stockholm, Sweden. This "Summary For Policymakers" lays out a powerful scientific case that significant climate change with severe impacts is already occurring, humans are mostly responsible, the pace of climate change is expected to accelerate, and we can make choices to cut emission of carbon dioxide and other heat-trapping gases that will limit the damage.

Q: How much has the planet warmed, and what has caused the warming?
The report documents that Earth's surface temperature warmed by 0.85°C (1.5°F) between 1880 - 2012. Two-thirds of this warming (0.6°C, 1.1°F) came after 1950. Human-emitted heat-trapping gases likely were responsible for 0.5 - 1.3°C of this post-1950 warming, while human-emitted aerosol particles reflected away sunlight and likely caused cooling (-0.6° - 0.1°C change in temperature.) Climate change due to variations in solar energy, volcanic dust, and natural sources of heat-trapping greenhouse gases were likely responsible for a small -0.1° - 0.1°C change in temperature since 1950. The sun was in a cool phase between 1978 - 2011, and the report estimates that lower solar output cooled Earth's climate slightly during this period. The influence of cosmic rays on climate over the past century was to weak to be detected, they said. In short, the report shows little support for a significant natural component to global warming since 1950. In fact, natural effects may well have made Earth cooler than it otherwise would have been. The report says that "The best estimate of the human-induced contribution to warming is similar to the observed warming over this period." In other words, close to 100% of the observed warming is due to humans.

Figure 1. The changing view of the IPCC's assessment reports on the human contribution to climate change.

Q: How have the IPCC reports changed through time?
1990: The report did not quantify the human contribution to global warming.

1995: "The balance of evidence suggests a discernible human influence on climate."

2001: Human-emitted greenhouse gases are likely (67-90% chance) responsible for more than half of Earth's temperature increase since 1951.

2007: Human-emitted greenhouse gases are very likely (at least 90% chance) responsible for more than half of Earth's temperature increase since 1951.

2013: Human-emitted greenhouse gases are extremely likely (at least 95% chance) responsible for more than half of Earth's temperature increase since 1951. This is the same confidence that scientists have in the age of the universe, or that cigarettes are deadly, according to an excellent AP article published this week by Seth Borenstein.

Q: Did the new report change the plausible range of global warming?
A. Yes. The "climate sensitivity" is defined as how much the planet would warm if the amount of atmospheric CO2 doubled. A variety of studies have arrived at very different estimates of the exact CO2 sensitivity of the climate, and the 2007 IPCC report gave a range of the most plausible values: 2 to 4.5ºC, with 3ºC deemed the most likely value. Recent research indicates that a sensitivity as low as 1.5ºC may be possible, so the IPCC widened the range of the most plausible values: 1.5 to 4.5ºC. The new lower limit of 1.5ºC is a best-case scenario that appears no more likely than the high end of 4.5ºC. Furthermore, even the lowest sensitivity scenario would not negate the need for emissions reductions. Current trends show that emissions are on track to increase far beyond doubling, which would create dangerous temperature rise even in a low-sensitivity climate. (Note that they give a small but worrisome possibility--0 to 10% chance--that the climate could warm by more than 6ºC for a doubling of CO2.)

Figure 2. Average of NASA's GISS, NOAA"s NCDC, and the UK Met Office's HadCRUT4 monthly global surface temperature departures from average, from January 1970 through November 2012 (blue), with linear trends applied to the time frames Jan '70 - Oct '77, Apr '77 - Dec '86, Sep '87 - Nov '96, Jun '97 - Dec '02, Nov '02 - Nov '12. Climate change skeptics like to emphasize the shorter term fluctuations in global temperatures (blue lines) and ignore the long-term climate trend (red line.) The global surface temperature trend from January 1970 through November 2012 (red line) is +0.16°C (+0.29°F) per decade. Image credit:

Q: What does the IPCC say about the "speed bump" in surface global warming over the past 10 - 15 years?
Much attention has been given in the press to the fact that the rate of surface warming over the past fifteen years has been slower than during previous decades. The report notes that due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012) of 0.05 °C per decade, which begins with a strong El Niño, is smaller than the rate calculated since 1951 of 0.12 °C per decade. However, the recent slow-down in surface warming is likely to be a mere "speed bump" on the highway of global warming, caused by natural variability. We have seen such "speed bumps" before, as well as short, sharp downhill stretches where surface warming speeds up. For example, climate scientist Stefan Rahmstorf writes at that "the warming trend of the 15-year period up to 2006 was almost twice as fast as expected (0.3°C per decade), and (rightly) nobody cared. We published a paper in Science in 2007 where we noted this large trend, and as the first explanation for it we named “intrinsic variability within the climate system”. Which it turned out to be." Physics demands that the massive amounts of heat-trapping carbon dioxide humans have dumped into the atmosphere must cause significant warming, but the chaotic complexity of the system is expected to obscure the magnitude of the long-term trend on time scales of a few years to a decade. The attention being to this latest "speed bump" on the highway of global warming is a direct result of a well-funded PR effort by the fossil fuel industry. One has to look at the total warming of the atmosphere, oceans, land, and ice to judge the true progress of global warming, not just the surface temperature. There has been no slowdown in total global warming when we regard this entire system, as I argued in a post earlier this year. More than 90% of the energy of global warming goes into the oceans, and the reason for the relative lack of surface warming this decade is that more heat than usual is being stored in the oceans. That heat will be released to the atmosphere at some point, removing the "speed bump".

The new IPCC report says that there is medium confidence that the "speed bump" in surface warming is due in roughly equal measure to natural multi-year unpredictable variability in the weather, and to changes in the amount of sunlight reaching the surface due to volcanic eruptions and the downward phase of the current solar cycle. Most of the climate models do not reproduce this lower surface warming rate during the past 10 - 15 years. There is medium confidence that this difference between models and observations is due to natural climate variability that is impossible to predict (for example, the El Niño/La Niña cycle), with possible contributions from the models' inadequate handling of volcanic eruptions, changes in solar output, and changes in light-reflecting aerosol particles, and, in some models, a too-strong response to heat-trapping gases. For an explanation of why arguments about the global warming “slowdown” are misleading and should not offer any consolation, see this explainer from Skeptical Science and this one from the Union for Concerned Scientists.

Q: What does the IPCC say about drought?
A: Drought and reduction in water availability due to decreased mountain snow and ice is the greatest threat civilization faces from climate change, since it attacks the two things we need to live--water and food. Unfortunately, the report makes no mention of drought in the text, and we will have to wait for the March 2014 release of the "impacts" portion of the report to hear more about the threat drought poses to society. Today's report does mention drought in one of their two tables, giving “low confidence”--a 20% chance--that we have already observed a human-caused increase in the intensity and/or duration of drought in some parts of the world. This is a reduction in confidence from the 2007 report, which said that it was more likely than not (greater than 50% chance.) However, the forecast for the future is the same as in the 2007 report: we are likely to see dry areas get dryer due to human-caused climate change by 2100. In particular, there is high confidence (80%) in likely surface drying in the Mediterranean, Southwest U.S., and Southern Africa by 2100 in the high-end emissions scenario (RCP8.5), in association with expected increases in surface temperatures and a shift in the atmospheric circulation that will expand the region of sinking air that creates the world's greatest deserts.

Q: What does the IPCC say about sea level rise?
A: Global average sea level has risen 7.5" (19 cm) since 1901. Sea level has accelerated to 1.5" (3.2 cm) per decade over the past 20 years--nearly double the rate of rise during the 20th century. The report projects that sea level will rise by an extra 0.9 - 3.2' (26 to 98 cm) by 2100. While the maximum sea level rise expected has gone up since the 2007 report, when the IPCC did not even consider melt from Greenland and Antarctica because of the primitive state of glacier science then, the new upper bound (3.2') is still is a very conservative number. IPCC decided not to include estimates from at least five published studies that had higher numbers, including two studies with rises of 2 meters (6.6 feet.) This is in contradiction to NOAA's December 2012 U.S. National Climate Assessment Report, which has 6.6 feet (2.0 meters) as its worst-case sea level rise scenario for 2100. Even this number may be too low; at a presentation Thursday in New York City for Climate Week, glaciologist Dr. Jason Box, who knows as much about Greenland's ice sheets as any person alive, explained that Greenland's contribution to global sea level rise doubled over the past ten years. If Greenland's melt rate continues to double every ten years until 2100, Greenland alone will contribute 4.6' (1.4 meters) of global sea level rise, he said. If the doubling time becomes every nine years, then Greenland will cause 16.4' (5 meters) of sea level rise by 2100. His best-guess number for global sea level rise by 2100 is 4.7' (1.5 meters), but warns that our models used to predict melting of ice of Greenland have large unknowns.

Long-term sea level rise is expected to be much greater. The IPCC report states with "very high confidence" that 119,000 - 126,000 years ago, during the period before the most recent ice age, sea levels were 16 - 33 feet (5 - 10 meters) higher than at present. Melting of Greenland "very likely" contributed 1.4 - 4.3 meters of this rise, with additional contributions coming from Antarctica. Temperatures at that time weren't more than 2°C warmer than "pre-industrial" levels during that period. Two of the four scenarios used for the report project we will exceed 2°C of warming by 2100, with "high confidence", raising the possibility that we could see sea level rises of many meters over time scales of 1,000 years or so. The report expects sea level rise reach 3.3 - 9.8' (1 - 3 meters) by 2300, assuming CO2 levels rise above 700 ppm (close to what the higher-end RCP6.0 scenario prescribes.)

Q: What does the IPCC say about ocean acidity?
A: The world's oceans have seen a 26% increase in acidity since the Industrial Revolution, as the average pH has dropped from 8.2 to 8.1. Under all report scenarios, the acidification of the world's oceans will increase, with the pH falling by another 0.06 - 0.32 units. According to a 2012 study in Science, the current acidification rate is likely the fastest in 300 million years, and "may have severe consequences for marine ecosystems."

Q: How about hurricanes?
A: The new report gives “low confidence”--a 20% chance--that we have observed a human-caused increase in intense hurricanes in some parts of the world. This is a reduction from the 2007 report, which said that it was more likely than not (greater than 50% chance.) The IPCC likely took note of a landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists, which concluded that the U.S. has not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes." The 2013 IPCC report predicts that there is a greater than 50% chance (more likely than not) that we will see a human-caused increase in intense hurricanes by 2100 in some regions; this is a reduction from the 2007 report, which said this would be likely (66% chance or higher.)

Q: How about extreme weather events?
"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 have decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia, and Australia. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency or intensity of heavy precipitation events has likely increased in North America and Europe." The report made no mention of tornadoes and severe thunderstorms, since the uncertainties of how they have behaved in the past and how climate change might affect them in the future are too great.

Q: What does the IPCC say about a "Day After Tomorrow" scenario?
A: In the disaster movie "The Day After Tomorrow", the Atlantic Meridional Overturning Circulation (AMOC)--the ocean current system of which the Gulf Stream Current is a part of--collapses, causing a rapid and extreme change in climate. A collapse of the AMOC is very unlikely (0 - 10% chance) before 2100 according to the report, but cannot be ruled out beyond the 21st century. A weakening of the AMOC by about 11 - 34% by 2100 is expected in the moderate RCP4.5 scenario, where CO2 levels reach 538 ppm in 2100. However, these odds assume that Greenland will dump a relatively modest amount of fresh water into the North Atlantic by 2100. If the higher-end sea level rise estimates that the IPCC did not consider as plausible come true, the AMOC will likely slow down much more, with a higher chance of collapse this century. No slow-down in the AMOC has been observed yet, according to the report.

As I read though the report, digesting the exhaustive list of changes to Earth's atmosphere, oceans, and ice that have occurred over the past few decades, I was struck by how the IPCC report reads like lab results from a sick hospital patient. The natural systems that civilization depends upon to thrive have been profoundly disturbed, and the forecast for the future reads like a medical diagnosis for an overweight smoker with a heart condition: unless the patient makes major lifestyle changes, the illness will grow far worse, with severe debilitation or death distinct possibilities. We can and we must make the huge effort to turn things around. Oil and natural gas are the energy technologies of the 20th century. Coal is the energy technology of the 19th century. We have countless innovative and dedicated people ready to move us to the energy technology of the 21st century; I heard three of them speak last night at the Climate Week event I am at, and they really gave me some needed hope that we can turn things around. We must elect new leaders and pressure our existing leaders to take the strong actions needed to advance us into a new, 21st century energy economy. You can all help make it so!

Jeff Masters

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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather

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Landmark 2013 IPCC Report: 95% Chance Most of Global Warming is Human-Caused

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased." Thus opens the landmark 2013 Intergovernmental Panel on Climate Change (IPCC) report issued today. Working without pay, hundreds of our most dedicated and talented ...

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