Working Group I
Findings of Note
AR5 reflects advances in science that point to increased levels of certainty on issues already raised and provides greater detail on measurements and projections. The most significant findings include:
Increased certainty on humans’ role. (more) The AR4 stated it was “very likely” — a figure that corresponds to at least 90% certainty — that human greenhouse gas pollution has caused more than half of observed warming. Scientists are now more certain than ever that observed warming can be attributed primarily to human activities such as deforestation and greenhouse gas pollution. The AR5 states with 95% certainty that humans have caused more than half of observed warming since 1951. Furthermore, the estimate of the human component of warming has increased over time while the estimate of the influence of natural factors has decreased.
Accelerating impacts of climate change. (more) The AR5 shows several indicators of climate change are advancing faster than originally thought.
- The melting of ice sheets over the past decade is happening several times faster than it was in the ‘90s. Glacial melt has accelerated as well.
- The rate of retreat of Arctic sea ice has increased from the AR4 estimate.
- Sea levels rose almost twice as fast from 1993 to 2010 as they did from 1901 to 2010.
Ocean acidification. (more) The AR5 affirms that due to excess carbon dioxide in the atmosphere, the pH of seawater has decreased. This poses risks to the development of many shellfish and other forms of ocean life as well as to the humans who depend on oceans for their livelihood.
Widened sensitivity to CO2. (more) The climate’s sensitivity to CO2 is characterized in different ways. One key way is to describe how much the planet would warm if the amount of atmospheric CO2 doubled. AR5 has expanded the range of possible values at the low end, from 2 – 4.5ºC to 1.5 – 4.5ºC. A lower sensitivity means it might take slightly longer before the worst impacts take place, but emissions reductions will still be necessary to prevent severe climate change impacts.
Short-term slowing of surface warming. (more) The AR5 mentions a reduced rate of surface warming over the past 15 years. Possible causes include emission of light-blocking aerosols by developing countries as well as the redirection of heat to the deep ocean. Research indicates that total global heat content is rising without pause, though, and surface warming is likely to resume when the ocean heat cycles back into the atmosphere.
These updates add to many of the conclusions that remain unchanged from the AR4, including the target of restricting warming to less than 2ºC. Projections show that this target is within reach, but only if world leaders follow through on significant emissions cuts by 2050.
Increased Certainty on Humans' Role
Scientists have become more certain of humans’ responsibility for climate change in each successive IPCC report. The AR4 stated it was “very likely” that human-generated greenhouse gas emissions caused more than half of observed warming, which they translated to at least 90% certainty. AR5 upped this confidence to 95%. New research indicates that since 1951, all of the observed warming may have come from human activities — even as natural forces and aerosols exerted a slight cooling influence
What the AR5 Says:
- “It is extremely likely (all emphases original) that more than half of the observed increase in global average surface temperature from 1951 to 2012 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.” (SPM-12)
- “There has been further strengthening of the evidence for human influence on temperature extremes since the SREX. It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heat waves in some locations.” (SPM-13)
- “It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s.” (SPM-13)
Peer-Reviewed Literature Cited in WG1:
Wigley, T. M. L. and B. D. Santer, 2013: A probabilistic quantification of the anthropogenic component of twentieth century global warming. Climate Dynamics, Vol. 40 no. 5-6 pp 1087-1102.
- There is a greater than 90 % probability that warming due to greenhouse gases only over 1950–2005 is larger than the total amount (not just “most”) of the observed warming. This is because the cooling effects of aerosols have substantially offset the greenhouse gas-induced warming.
Jones, Gareth et al., 2013: Attribution of observed historical near‒surface temperature variations to anthropogenic and natural causes using CMIP5 simulations. Journal of Geophysical Research: Atmospheres, Vol. 118 no. 10 pp 4001-4024. DOI: 10.1002/jgrd.50239
- Simulations that incorporate both human-caused and natural factors accurately model observed temperatures between 1860 and 2010. Simulations using only natural factors do not show as much warming as the amount observed.
Stott, Peter and Gareth Jones, 2012: Observed 21st century temperatures further constrain likely rates of future warming. Atmospheric Science Letters, Vol. 13 no. 3 pp 151-156. DOI: 10.1002/asl.383
- Analysis of observed near-surface temperatures to 2010 demonstrates the human influence on climate has strengthened over the first decade of the 21st century. Estimates of future warming rates are now more specific than they were a decade ago.
Lockwood, Mike, 2012: Solar influence on global and regional climates. Surveys in Geophysics, Vol. 33 no. 3-4, pp 503-534.
- The best estimates of the solar influence on the global mean air surface temperature show relatively small effects when compared to human-induced changes.
Jones et al., 2011: Detecting the influence of fossil fuel and bio-fuel black carbon aerosols on near surface temperature changes. Atmospheric Chemistry and Physics, Vol. 11 pp 799-816. DOI:10.5194/acp-11-799-2011
- Fossil fuel and biofuel black carbon (soot) is found to have a detectable contribution to the warming over the last 50 years of the 20th century. However, this influence is small compared to that from greenhouse gas emissions.
Chrystidis et al., 2010: Probabilistic estimates of recent changes in temperature: a multi-scale attribution analysis. Climate Dynamics, Vol. 34 no. 7-8 pp 1139-1156.
- Human influence is estimated to have more than doubled the likelihood of warming in all regions considered except central North America, where results were inconclusive.
Lockwood, Mike, 2008: Recent changes in solar outputs and the global mean surface temperature. III. Analysis of contributions to global mean air surface temperature rise. Proceedings of the Royal Society A, Vol. 64 no. 294 pp 1387-1404. DOI: 10.1098/rspa.2007.0348
- The contribution of solar variability to the temperature trend since 1987 is small and downward. The best estimate is that the anthropogenic factors contribute 75% of the rise since 1987.
New Peer-Reviewed Literature Since the WGI Deadline:
Santer et al., 2013: Human and natural influences on the changing thermal structure of the atmosphere. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1305332110
- Climate models are used to depict a climate free of human influence, and this is compared to satellite data. Without human influence, models do not reproduce the observed latitude/altitude pattern of temperature change.
This site may be too technical for some but provides links to peer-reviewed studies as well as useful graphics and charts. The authors frame their articles as responses to common questions, such as Is the Sun causing global warming? They also summarize the overall evidence attributing global warming to humans.
Climate Reality Project
A useful resource for when the simplest possible explanation is needed.
Environmental Defense Fund
A straightforward summary of the relationship between CO2 and global warming with strong conclusions and good graphs and stats.
Union of Concerned Scientists
A more in-depth summary with peer-reviewed citations and internal cross-links to background information on other relevant topics. Includes an FAQ on aerosols and warming in addition to especially great graph showing the trajectories of different climate forcings.
A moderately technical explanation of how models are used to take known climate-influencing factors and estimate the contribution each one makes to global warming.
Accelerating Impacts of Climate Change
The rates of change attributed to several impacts have accelerated since AR4. For example, the melting of sea ice, glaciers, and ice sheets has accelerated, as has sea level rise.
The AR4 stated with “high” confidence that sea level rise is accelerating, and the AR5 adds detail to this statement.
The AR5 also notes that the melt rate of the Greenland and Antarctic ice sheets was several times faster in the past decade than in the 1990s. The melt rate of glaciers in the past 20 years was faster than it was prior to 1993.
What the AR5 Says:
- “The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of ice sheets, was very likely 226 [91 to 361] Gt yr-1 over the period 1971-2009, and very likely 275 [140 to 410] Gt yr-1 over the period 1993-2009.” (SPM-5)
- “The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [-6 to 74] Gt yr-1 over the period 1992-2001 to 215 [157 to 274] Gt yr-1 over the period 2002-2011.” (SPM-5)
- “The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [-37 to 97] Gt yr-1 over the period 1992-2001 to 147 [72 to 221] Gt yr-1 over the period 2002-2011.” (SPM-5)
- “The annual mean Arctic sea ice extent decreased over the period 1979-2012 with a rate that was very likely in the range 3.5 to 4.1% per decade (range of 0.45 to 0.51 million km2 per decade), and very likely in the range 9.4 to 13.6% per decade (range of 0.73 to 1.07 million km2 per decade) for the summer sea ice minimum (perennial sea ice).” (SPM-6)
- “It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm yr-1 between 1901 and 2010, 2.0 [1.7 to 2.3] mm yr-1 between 1971 and 2010 and 3.2 [2.8 to 3.6] mm yr-1 between 1993 and 2010.” (SPM-6)
Melting Glaciers and Ice Sheets, Peer-Reviewed Literature Cited in WG1:
Gardner, A. et al., 2013: A reconciled estimate of glacier contributions to sea level rise: 2003 to 2009, Science, Vol. 340 no. 6134, pp 852-857. DOI: 10.1126/science.1234532
- Standardization of many existing estimates shows that the world’s glaciers are losing about 259 gigatons of mass per year. Together, the melting of glaciers and ice sheets is responsible for around a third of observed sea level rise between 2003 and 2009.
Gardner, A. et al., 2011: Sharply increased mass loss from glaciers and ice caps in the Canadian Arctic Archipelago. Nature, 473, pp 357–360. DOI:10.1038/nature10089
- Melting Canadian Arctic ice is largest contribution to eustatic (melting) sea level rise besides Greenland and Antarctica. The rate of mass loss in Canadian Arctic sharply increased between 2004 and 2009.
Rignot, E. et al., 2011: Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, Vol 38, issue 5. DOI: 10.1029/2011GL046583
- Two different techniques show an accelerating rate of mass loss from the Greenland and Antarctic ice sheets. If the trend continues, ice sheets will be dominant force raising sea levels in 21st century.
Retreat of Arctic Sea Ice, Peer-Reviewed Literature Cited in WG1:
Comiso, Josefino C., 2012: Large decadal decline of the Arctic multiyear ice cover. J. Climate, 25, pp 1176–1193. DOI: http://dx.doi.org/10.1175/JCLI-D-11-00113.1
- Arctic sea ice extent trends are strongly negative. Overall sea ice extent is decreasing more than 12% per decade, while thick multi-year ice extent is decreasing even faster at more than 15% per decade.
Wadhams, P et al., 2011: Arctic sea ice thickness characteristics in winter 2004 and 2008 from submarine sonar transects. Journal of Geophysical Research: Oceans, Vol 116, C8. DOI: 10.1029/2011JC006982
- Submarine data shows a 32% decrease in mean ice draft (the amount of ice extending below the water surface) occurred over 31 years.
Kwok, R. et al., 2009: Thinning and volume loss of the Arctic Ocean sea ice cover: 2003-2008. Journal of Geophysical Research: Oceans, Vol 114, C7. DOI: 10.1029/2009JC005312
- Since 2005, multi-year ice coverage has decreased more than 42%, and winter multi-year ice volume has decreased more than 40%. During the period from 2003-2008, ice thinned more than 0.6m.
Rothrock, D.A. et al., 2008: The decline in arctic sea-ice thickness: Separating the spatial, annual, and interannual variability in a quarter century of submarine data. Journal of Geophysical Research: Oceans, Vol 113, C5. DOI: 10.1029/2007JC004252
- Naval submarine data from 1975-2000 show a 1.25m decrease in annual mean ice thickness in the Arctic Ocean.
Sea Level Rise Peer-Reviewed Literature Cited in WG1:
Tebaldi, C., Strauss, B. H., & Zervas, C. E., 2012: Modeling sea level rise impacts on storm surges along US coasts. Environmental Research Letters, 7(1), 014032.
- Models show that even small amounts of sea level rise contribute to greater impact of storm surges. By mid-century, some locations will annually experience high water levels that are currently considered a rarity (that is, they have only a 1% chance of occurring in any given year).
Church, J. White, N., 2011: Sea-level rise from the late 19th to the early 21st century. Surveys in Geophysics, Volume 32 Issue 4-5, pp 585-602. DOI: 10.1007/s10712-011-9119-1
- Sea level data back to 1880 show that sea levels have been rising, and the rate accelerating significantly.
Woodworth, P. L. et al., 2009: Evidence for the accelerations of sea level on multi-decade and century timescales. International Journal of Climatology. Vol 29, issue 6, pp 777-789, DOI: 10.1002/joc.1771
- Multiple study findings show a trend of accelerating sea level rise.
Sea Ice, Ice Sheets, and Glaciers Resources:
National Snow & Ice Data Center
The National Snow & Ice Data Center provides a number of helpful resources, such as a quick fact sheet on ice sheets. The World Glacier Inventory is the most comprehensive data set on glaciers. They also offer up-to-date news and analysis on Arctic sea ice. They also offer a list of easy-to-use database for non-scientists, which contain links to a number of accessible datasets and a glossary of cryosphere terms.
International Arctic Research Center
The International Arctic Research Center, (IARC) out of the University of Alaska Fairbanks, is a collaboration project between multiple groups. They have research available, along with a basic overview of the Arctic and Real-Time Monitors plus educational materials for non-scientists.
Arctic Sea Ice Blog
Although written by a non-scientist, the Arctic Sea Ice Blog is a great source of information.
Sea Level Rise Outside Resources
Surging Seas: Sea level rise analysis is a tool that lets the user see (at the state, city or zip code level) how much land and how many people will be impacted by various levels of sea level rise.
The AR4 stated that the pH of seawater has decreased by 0.1 since the beginning of the industrial era. This may not seem like much, but the change corresponds to a 26% increase in the hydrogen ions that are the basis for determining pH, and it is enough to impede the development of some species of shellfish and corals. AR5 confirms this finding and adds new projections that show the increase of ocean acidification under differing emissions scenarios.
What the AR5 Says:
- “The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.” (SPM 7)
- “Further uptake of carbon by the ocean will increase ocean acidification.” (SPM 19)
- “The pH of ocean surface water has decreased by 0.1 since the beginning of the industrial era (high confidence), corresponding to a 26% increase in hydrogen ion concentration.” (SPM 8)
Peer-Reviewed Literature for Ocean Acidification Cited in WG1:
Byrne, R. H., S. Mecking, R. A. Feely, and X. W. Liu, 2010: Direct observations of basin-wide acidification of the north pacific ocean. Geophysical Research Letters, 37. doi:10.1029/2009gl040999
- Research shows the first basin-wide direct observations of recently declining pH, along with estimates of anthropogenic and non-anthropogenic contributions to that signal.
Doney, S. C., V. J. Fabry, R. A. Feely, and J. A. Kleypas, 2009: Ocean acidification: The other co2 problem. Annual Review of Marine Science, 1, pp 169-192. doi:10.1146/annurev.marine.010908.163834
- Rising atmospheric CO2, primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The rate will accelerate over this century unless future CO2 emissions are curbed dramatically.
Feely, R. A., S. C. Doney, and S. R. Cooley, 2009: Ocean acidification: Present conditions and future changes in a high-co2 world. Oceanography, 22, pp 36-47
- Elevated CO2 will cause substantial reductions in surface water carbonate ion concentrations in terms of either absolute changes or fractional changes relative to pre-industrial levels.
Fabry, V. J., B. A. Seibel, R. A. Feely, and J. C. Orr, 2008: Impacts of ocean acidification on marine fauna and ecosystem processes. Ices Journal of Marine Science, 65, pp 414-432. doi:10.1093/icesjms/fsn048
- Ocean acidification and the synergistic impacts of other anthropogenic stressors provide great potential for widespread changes to marine ecosystems. For example, acidification affects the ability of mollusks and invertebrates to properly form their shells and exoskeletons.
New Peer-Reviewed Literature on Ocean Acidification Since the WGI Deadline:
- Ocean acidification is linked to a reduced concentration of aragonite, a mineral that also occurs in mollusk shells. The melting of ice sheets, which dilutes ocean water with fresh water, also plays a role. The mineral is critical to the survival of mollusks, whose shells are composed of aragonite.
Ocean-Based Food Security Threatened in a High CO2 World ranks nations based on the seafood security hardships they may experience by the middle of this century due to changing ocean conditions from climate change and ocean acidification.
Ocean Acidification: The Untold Stories illustrates how the changing acidity of the oceans threatens to throw off the delicate chemical balance marine life depends on for survival. It highlights the fact that although the attention that ocean acidification has received has focused mainly on corals, the impacts are going to be far-reaching throughout the oceans.
Acid Test: Can We Save Our Oceans from CO2? gives a general overview of the science behind ocean acidification, and focuses on how coral reefs will bear the brunt soonest.
Washington State Blue Ribbon Commission on Ocean Acidification
Washington Governor Gregoire created the Washington State Blue Ribbon Commission on Ocean Acidification to direct state agencies to take steps to reduce the pollutants that contribute to acidification. Its 2012 report outlines the panel’s specific recommendations for addressing ocean acidification.
Widened Range of Sensitivity to CO2
The climate’s sensitivity to CO2 is a way to describe how much the planet would warm if the amount of atmospheric CO2 doubled. A variety of studies have arrived at differing estimates on the exact CO2 sensitivity of the climate, so the AR4 gave a range of the most plausible values: 2 to 4.5ºC, with 3ºC deemed the most likely value.
The AR5 indicates that a sensitivity as low as 1.5ºC is possible, but this is a best-case scenario that appears no more likely than the high end, 4.5ºC. Furthermore, even the lowest sensitivity scenario doesn’t 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.
Peer-Reviewed Literature on Climate Sensitivity Cited in WG1:
Knutti, Reto and Gabriele C. Hegerl, 2008: The equilibrium sensitivity of the Earth’s temperature to radiation changes. Nature Geoscience, Vol. 1 pp 735-743. DOI:10.1038/ngeo337
- Science has a long history of the search for a narrowed range of possible climate sensitivity values, and the range has not narrowed very much even after many years of work. The most likely value remains 3ºC.
Annan, J.D., and J. C. Hargreaves, 2011: On the generation and interpretation of probabilistic estimates of climate sensitivity. Climatic Change, Vol. 104 no. 3-4 pp 423-436.
- Some studies promoting extremely high sensitivity may have used questionable assumptions. Modified assumptions led to a new estimate: 95% chance that climate sensitivity is less than 4ºC.
Zickfeld, Kirsten et al., 2010: Expert judgments about transient climate response to alternative future trajectories of radiative forcing. Proceedings of the National Academy of Sciences of the U.S.A., Vol. 107 no. 28 pp 12451-12456. DOI: 10.1073/pnas.0908906107
- Interviews with 14 experts attempt to quantify uncertainty surrounding climate sensitivity. Ten of the 14 experts estimated a greater than 17% chance that sensitivity exceeds 4.5ºC. Most experts agreed that over the next 20 years, research will be able to achieve only modest reductions in their degree of uncertainty.
Schmittner, Andreas et al., 2011: Climate sensitivity estimated from temperature reconstructions of the last glacial maximum. Science, Vol. 334 no. 6061 pp 1385-1388. DOI: 10.1126/science.1203513
- Reconstructions of prehistoric temperatures show a reduced estimate of climate sensitivity. Authors posit a median value of 2.3ºC, and a likely range of 1.7-2.6ºC.
Olson, Roman et al., 2012: A climate sensitivity estimate using Bayesian fusion of instrumental observations and an Earth System model. Journal of Geophysical Research: Atmospheres, Vol. 117 no. 27. DOI: 10.1029/2011JD016620
- A combination of modeled and observed temperatures point toward a climate sensitivity estimate with a likely range of 1.8 to 4.9ºC, and a most likely value of 2.8ºC.
Huber, Markus et al., 2011: Constraints on climate sensitivity from radiation patterns in climate models. American Meteorological Society. Vol. 24 pp 1034-1052. DOI: http://dx.doi.org/10.1175/2010JCLI3403.1
- A variety of models find that sensitivity values below 1.7ºC are not consistent with observed patterns. Aggregation of the various datasets used shows that sensitivity is not likely to be below 2.9ºC, and values exceeding 4.5ºC cannot be excluded.
This FAQ on climate sensitivity explains the significance of the number, as well as why low sensitivity values do not refute the need for greenhouse gas reduction.
This blog post lays out a brief comparison between different ways of estimating sensitivity and explains common misinterpretations by the media. Another post reiterates this summary and provides a good graphic to show the convergence of many recent estimates of the sensitivity range.
A series of graphics illustrates the many different estimates of climate sensitivity ranges that have been published over the years and shows how they’ve remained in a similar range since early estimates in the 1970s.
In another post on the Guardian’s 97% blog, Dana Nucitelli of Skeptical Science explains the basics of climate sensitivity and the various feedbacks that affect estimates.
This post provides a more scientific explanation of climate sensitivity that goes into detail about the various methodologies used to calculate estimates as well as provides links to recent papers.
Short-Term Slowing of Surface Warming
The AR5 mentioned a reduced rate of surface warming over the past decade. Aerosols are known to exert a cooling influence on the climate, and some developing countries such as China are increasing their aerosol emissions as their industrial output rises. Volcanoes also emit cooling aerosols.
Another possible factor slowing the surface warming is the redirection of heat into the deep ocean. Historically, the deepest reaches of the ocean have been difficult to measure, but new research techniques show that heat in the deep ocean is increasing. This warming is likely to be released back to the surface as natural ocean cycles continue.
Many interactions between natural and human-caused factors have caused the rate of surface warming to rise and fall several times over the past century, so the present period is consistent with a warming world overall.
What the AR5 says:
“In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability (see Figure SPM. 1). 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 in the past 15 years (1998-2012; 0.05 [-0.05 to +0.15] ºC per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951-2012; 0.12 [0.08 to 0.14] ºC per decade).” (SPM-3)
“The forcing from stratospheric volcanic aerosols can have a large impact on the climate for some years after volcanic eruptions. Several small eruptions have caused a RF of -0.11 [-0.15 to -0.08] W m-2 for the years 2008-2011, which is approximately twice as strong as during the years 1999-2002.” (SPM-9)
“The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend.” (SPM-10)
“There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability.” (SPM-10)
“Warming will continue to exhibit interannual-to-decadal variability and will not be regionally uniform.” (SPM-15)
Peer-Reviewed Literature Cited in WG1:
Kaufman, Robert et al., 2011: Reconciling anthropogenic climate change with observed temperature 1998-2008. Proceedings of the Natural Academy of Sciences of the U.S.A., Vol. 108 no. 29 pp 11790-11793. DOI: 10.1073/pnas.1102467108
- Declining solar activity as part of a normal eleven-year cycle, a change from an El Niño to a La Niña, and rapid growth in short-lived sulfur emissions partially offset human-caused warming influences. Thus, recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature and human and natural influences.
Foster, Grant and Steven Rahmstorf, 2011: Global temperature evolution 1979-2010. Environmental Research Letters, Vol. 6, no. 4. DOI:10.1088/1748-9326/6/4/044022
- Five major temperature series all show consistent warming trends from 1979 to 2010. When the data are adjusted to remove the impact of known natural factors, such as volcanic aerosols, El Niño, and solar variability, the warming trend becomes even more pronounced and is steady over the whole time period.
Fyfe et al., 2010: Comparing variability and trends in observed and modeled global-mean surface temperature. Geophysical Research Letters, Vol. 37 no. 16. DOI: 10.1029/2010GL044255
- When natural signals such as El Niño and volcanoes are removed from temperature trends, the statistical uncertainty in linear trends from 1950 to 2000 drops by about half.
Liebmann et al., 2010: Influence of Choice of Time Period on Global Surface Temperature Trend Estimates. Bulletin of the American Meteorological Society, Vol. 91 pp 1485-1491. DOI: http://dx.doi.org/10.1175/2010BAMS3030.1
- Global mean and land surface temperature changes over the past century are extremely unlikely to have occurred by chance. In contrast, short-term trends over less than a few decades are generally not statistically significant. This implies that decadal variability is a large factor in trends estimated over such short time periods.
New Peer-Reviewed Literature Since the WG1 Deadline:
Balmaseda, Magdalena et al., 2013: Distinctive climate signals in reanalysis of global ocean heat content. Geophysical Research Letters, Vol. 40 no. 9 pp 1754-1759. DOI: 10.1002/grl.50382
- Although warming of surface atmosphere and upper oceans has slowed, the deep oceans have absorbed 30% of all warming, contributing to an acceleration of overall global warming.
Cowtan and Way, 2013: Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends. Quarterly Journal of the Royal Meteorological Society. DOI: 10.1002/qj.2297
- The HadCRUT4 temperature series has a cool bias because it undersamples the Arctic, which is warming faster than the rest of the planet. New methods can help create more accurate reconstructions of temperature trends which include the undersampled regions, and show that global average temperatures may have warmed more than previously thought over thought over the past decade.
This guide, with links to other explanatory material and peer-reviewed studies, shows how global warming is continuing despite the slowdown in warming of atmospheric surface temperatures.
This post has some great graphics showing the role of ocean heat in causing the apparent slowdown, as well as what temperature trends look like with the dampening impacts of natural variation removed. Another similar post adds a breakdown of many relevant forcings and notes whether they warm or cool the climate.
This article shows how natural variations have caused many so-called “pause” periods in atmospheric temperatures in the past, but show that warming has always resumed. The article also notes that accelerating sea level rise points to unabated warming in the oceans.
A clever video shows how variation can exist in the context of an overarching trend, using the metaphor of a dog owner walking a dog. Likewise, the historic progress of surface warming shows significant variation, while the overall trend is upward.
A science blogger does a great post on how strategically choosing the segment of data to show has a large impact on whether a trend is apparent or not. This is an example of what is commonly criticized as “cherry-picking.”