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 (see Figure SPM-1). 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. {2.3, 6.4, 7.3}
Carbon dioxide is the most important anthropogenic greenhouse gas (see Figure SPM-2). The global
atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to
379 ppm3 in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural
range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. The annual carbon
dioxide concentration growth-rate was larger during the last 10 years (1995 – 2005 average: 1.9 ppm per
year), than it has been since the beginning of continuous direct atmospheric measurements (1960–2005
average: 1.4 ppm per year) although there is year-to-year variability in growth rates.
The global atmospheric concentration of methane has increased from a pre-industrial value of about 715
ppb to 1732 ppb in the early 1990s, and is 1774 ppb in 2005. The atmospheric concentration of methane in
2005 exceeds by far the natural range of the last 650,000 years (320 to 790 ppb) as determined from ice
cores. Growth rates have declined since the early 1990s, consistent with total emissions (sum of
anthropogenic and natural sources) being nearly constant during this period. It is very likely6 that the
observed increase in methane concentration is due to anthropogenic activities, predominantly agriculture
and fossil fuel use, but relative contributions from different source types are not well determined. {2.3, 7.4}
The combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is +2.30
[+2.07 to +2.53] W m-2, and its rate of increase during the industrial era is very likely to have been
unprecedented in more than 10,000 years (see Figures SPM-1 and SPM-2). The carbon dioxide radiative
forcing increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200
years. {2.3, 6.4}
Significant anthropogenic contributions to radiative forcing come from several other sources. Tropospheric
ozone changes due to emissions of ozone-forming chemicals (nitrogen oxides, carbon monoxide, and
hydrocarbons) contribute +0.35 [+0.25 to +0.65] W m-2. The direct radiative forcing due to changes in
halocarbons8 is +0.34 [+0.31 to +0.37] W m-2. Changes in surface albedo, due to land-cover changes and
deposition of black carbon aerosols on snow, exert respective forcings of -0.2 [-0.4 to 0.0] and +0.1 [0.0 to
+0.2] W m-2. Additional terms smaller than +0.1 W m-2 are shown in Figure SPM-2. {2.3, 2.5, 7.2}
Observed rate of sea level rise and estimated contributions from different sources. {5.5, Table 5.3}
(very interesting)
Recent trends, assessment of human influence on the trend, and projections for extreme weather events for
which there is an observed late 20th century trend. {Tables 3.7, 3.8, 9.4, Sections 3.8, 5.5, 9.7, 11.2-11.9}
(very interesting)
Paleoclimate information supports the interpretation that the warmth of the last half century is
unusual in at least the previous 1300 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 metres of sea level rise. {6.4, 6.6}
uh oh...
It is very unlikely that climate changes of at least the seven centuries prior to 1950 were due to variability
generated within the climate system alone. A significant fraction of the reconstructed Northern Hemisphere
interdecadal temperature variability over those centuries is very likely attributable to volcanic eruptions and
changes in solar irradiance, and it is likely that anthropogenic forcing contributed to the early 20th century
warming evident in these records.
Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the
full effects of changes in ice sheet flow, because a basis in published literature is lacking. The projections
include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for
1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution
were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES
scenarios shown in Table SPM-2 would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but
understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper
bound for sea level rise. {10.6}
Thats funny. They don't know how high sea level is going to rise so they just say it will progress linearly, even though the temperatures and ice melts have not.
Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100.
Current models suggest ice mass losses increase with temperature more rapidly than gains due to
precipitation and that the surface mass balance becomes negative at a global average warming (relative to
pre-industrial values) in excess of 1.9 to 4.6°C. If a negative surface mass balance were sustained for
millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting
contribution to sea level rise of about 7 m. The corresponding future temperatures in Greenland are
comparable to those inferred for the last interglacial period 125,000 years ago, when paleoclimatic
information suggests reductions of polar land ice extent and 4 to 6 m of sea level rise.
"If you are a real seeker after truth, it's necessary that at least once in your life you doubt all things as far as possible"-Rene Descartes
"When you have excluded the impossible, whatever remains however improbable must be the truth"-Sherlock Holmes