The IPCC has released the Summary For
Policymakers of the Fourth Assessment Report. Some of the conclusions:
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely [defined as >90% probability] due to the observed increase in anthropogenic greenhouse gas concentrations
The equilibrium climate sensitivity is a measure of the climate system
response to sustained radiative forcing. It is not a projection but is
defined as the global average surface warming following a doubling of
carbon dioxide concentrations. It is likely [>66% chance] to be in the range 2 to
4.5°C with a best estimate of about 3°C, and is very unlikely [<10% chance] to be
less than 1.5°C. Values substantially higher than 4.5°C cannot be
excluded, but agreement of models with observations is not as good for
those values.Best estimates and likely ranges for globally average surface air
warming [by the end of the century] for six SRES emissions marker
scenarios are given in this assessment and are shown in Table
SPM-2. For example, the best estimate for the low scenario (B1) is
1.8°C (likely range is 1.1°C to 2.9°C), and the best estimate for the
high scenario (A1FI) is 4.0°C (likely range is 2.4°C to
6.4°C). Although these projections are broadly consistent with the
span quoted in the TAR (1.4 to 5.8°C), they are not directly
comparable.
Projected sea level rises range from 0.18-0.38m for scenario B1 to 0.26-0.59m for A1FI. But these exclude "future rapid dynamical changes in ice flow", so larger rises are possible.
In the sentence "MOST of the observed increase in globally averaged temperatures since the mid-20th century is very likely [defined as >90% probability] due to ......."
what does the word MOST mean ?:
1) The largest single source among several
2) More than 50 %
3) More than 90 %
4) Something else
If 1) is correct, what is the size of this source ?
Before we go kicking and bashing and accepting the Summary Report, how about we take a moment to thank the thousands of scientists who gave a large part of their last four years to bringing forth this document and all that will follow.
They are giants and we are forever in their debt.
The report says that it's likely (>67% chance) that more than 100% of the warming is caused by GHGs. They don't define "most" -- presumably it means >50%.
Well, the CAbots are still talking about HS shapes, so of course that means the AR4 is wrong.
Best,
D
Likely range from 1.1°C to 6.4°C? That's not an estimate, that's a dart board.
More than 100%?
Ben: I believe that what they are getting at is that there is cooling due to sulphate aerosols. Thus the warming that we have experienced is enough to offset the SA cooling and still provide the warming that we have measured. However maybe someone who has read the document could comment.
Followup to my previous post. That appears to be what it is saying (I decided that I had too much work to finish today so gave up and skimming the report).
Bottom of page 8:
It is likely that increases in greenhouse gas concentrations alone would have caused more warming than
observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise
have taken place. {2.9, 7.5, 9.4}
OK, so I guess somewhere in the AR4 document there is a quantitative assesment of studies related to volcanic and anthro aerosol effects including any cooling reflective effect and warming soot/albedo effects? And the results turned out to point in the direction of overall cooling due to aerosols? Even with warming observed over regions of anthro aerosol production today? Looking forward to reading about that...
"Likely range from 1.1°C to 6.4°C? That's not an estimate, that's a dart board."
Yes, predicting human behaviour is a dartboard.
If I understand your question correctly, it's on page 16.