Scientific consensus on climate change

"The sensitivity and generality of modern analytical systems make it quite unlikely that any currently significant greenhouse gases remain to be discovered."

"[D]ecadal average warming over each inhabited continent by 2030... is very likely to be at least twice as large as the corresponding model-estimated natural variability during the 20th century."

"Model experiments show that even if all radiative forcing agents were held constant at year 2000 levels, a further warming trend would occur in the next two decades at a rate of about 0.1°C per decade, due mainly to the slow response of the oceans. About twice as much warming (0.2°C per decade) would be expected if emissions are within the range of the SRES scenarios."

"Atmospheric concentrations of CFCs rose steadily following their first synthesis in 1928 and peaked in the early 1990s. Many other industrially useful fluorinated compounds... have very long atmospheric lifetimes, which is of concern... s (HFCs), which are replacing CFCs, have a greenhouse effect, but it is much less pronounced..."

"The IPCC’s conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community... Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past 20 years."

"Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilised."

"Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years."

"Since IPCC’s first report in 1990, assessed projections have suggested global average temperature increases between about 0.15°C and 0.3°C per decade for 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections..."

"Of the greenhouse gases that are directly influenced by human activity, the most important are carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons (CFCs). s released by human activities are also capable of influencing climate."

"Nitrous oxide (N2O) is formed by many microbial reactions in soils and waters, including those acting on the increasing amounts of nitrogen-containing fertilizers. Some synthetic chemical processes that release nitrous oxide have also been identified. Its concentration has increased approximately 13% in the past 200 years."

"[A]ssessments that examine only the next 100 years may well underestimate the magnitude of the eventual impacts."

"If a central estimate of climate sensitivity is used, about 40% of the predicted warming is due to the direct effects of greenhouse gases and aerosols. The other 60% is caused by s."

"Like carbon dioxide, (CH4) is more abundant in Earth’s atmosphere now than at any time during the 400,000 year long ice core record, which dates back over a number of glacial/interglacial cycles."

"es are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise."

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."

"Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 m of sea level rise."

"Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns..."

"For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected."

"Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the time scales required for removal of this gas from the atmosphere..."

"[N]ational policy decisions made now and in the longer-term future will influence the extent of any damage suffered by vulnerable human populations and ecosystems later in this century."

"About two-thirds of the current emissions of methane are released by human activities such as rice growing, the raising of cattle, coal mining, use of land-fills, and natural gas handling, all of which have increased over the past 50 years."

"Black carbon aerosols are end-products of the incomplete combustion of fossil fuels and biomass burning (forest fires and land clearing)."

"Water vapor feedback (the additional greenhouse effect accruing from increasing concentrations of atmospheric water vapor as the atmosphere warms)... is expected to increase the temperature response to increases in human induced greenhouse gas concentrations by a factor of 1.6. The ice-albedo feedback (the reduction in the fraction of incoming solar radiation reflected back to space as snow and ice cover recede) also is believed to be important. Together, these two feedbacks amplify the simulated climate response to the greenhouse gas forcing by a factor of 2.5."

"[H]eat stress and smog induced respiratory illnesses in major urban areas would increase, if no adaptation occurred."

"[R]isk increases with increases in both the rate and the magnitude of climate change."

"With higher sea level, coastal regions could be subject to increased wind and flood damage..."

"Recent "clean coal technologies" and use of low sulfur fuels have resulted in decreasing sulfate concentrations..."

"Besides greenhouse gases, human activity also contributes to the atmospheric burden of s, which include both sulfate particles and (soot)."

"Concentrations of (CO2) extracted from s drilled in Greenland and Antarctica have typically ranged from near 190 parts per million by volume (ppmv) during the ice ages to near 280 ppmv during the warmer "interglacial" periods like the present one that began around 10,000 years ago. Concentrations did not rise much above 280 ppmv until the Industrial Revolution. By 1958... they had reached 315 ppmv, and they are currently ~370 ppmv and rising at a rate of 1.5 ppmv per year... Human activities are responsible for the increase. The primary source, fossil fuel burning, has released roughly twice as much... as... required... for the observed increase. Tropical deforestation also has contributed to carbon dioxide releases during the past few decades. The excess... has been taken up by the oceans and land ."

"Human-induced warming and associated sea level rises are expected to continue through the 21st century."

"Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent as global mean surface temperature increases."

"Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years... The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture."

"At continental, regional and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones."

"Average arctic temperatures increased at almost twice the global average rate in the past 100 years."

"Global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice. Ice core data indicate that average polar temperatures at that time were 3°C to 5°C higher than present, because of differences in the Earth’s orbit."

"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

"[I]t is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone..."

"Anthropogenic forcing is likely to have contributed to changes in wind patterns, affecting extratropical storm tracks and temperature patterns in both hemispheres."

"Marine organisms will face progressively lower oxygen levels and high rates and magnitudes of ..."

"Most plant species cannot naturally shift their geographical ranges sufficiently fast to keep up with current and high projected rates of climate change in most landscapes; most small mammals and s will not be able to keep up at the rates projected... in flat landscapes in this century..."

"[N]egative impacts of climate change on crop yields have been more common than positive impacts..."

"Climate change will amplify existing risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development."

"The global ocean will continue to warm during the 21st century, with the strongest warming projected for the surface in tropical and Northern Hemisphere subtropical regions..."

"Many terrestrial, freshwater and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change..."

"This report distils, synthesizes and integrates the key findings of the three Working Group contributions—The Physical Science Basis, Impacts, Adaptation, and Vulnerability and Mitigation of Climate Change..."

"Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent."

"It is very likely that heat waves will occur with a higher frequency and longer duration. Occasional cold winter extremes will continue to occur."

"In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans."

"Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent (e.g., individual, community, company, country) affect other agents."

"It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales, as global mean surface temperature increases."