James Hansen Draft Article: Warming May Hit 6C, Not 3C, @ Doubled CO2 Levels - Grist

[hatrack]

EDIT

Variations of atmospheric CO2 occurring as a climate feedback on the time scale of the ice ages (Figure 3) can be ~100 ppm in 5000 years, or 0.02 ppm/year. This atmospheric change is due to a shifting of carbon among the atmosphere, ocean, soil and biosphere compartments within the surface carbon pool, a warmer climate driving more CO2 into the air. This natural glacial-interglacial variation of atmospheric CO2 is quite rapid in comparison with the geologic cycling of carbon between the Earth's crust and the surface carbon pool, which amounts to ~10**(-4) ppm/year of CO2, as discussed above.

These natural rates of atmospheric CO2 change must be compared with the human-caused growth of atmospheric CO2, which is now ~2 ppm/year (see below). Humans, indeed, are now in control of long-lived atmospheric GHGs. As a result it is important to investigate climate sensitivity for the case in which GHGs are specified as the forcing. The Charney climate sensitivity applies to this case under the assumption that slow feedbacks such as ice sheet area, vegetation distribution, and climate-induced GHG changes are not allowed to operate.

As a complement to the Charney climate sensitivity, let us derive the climate sensitivity that applies if these slow feedbacks are allowed to operate: we call this the "long-term" climate sensitivity. We can obtain this "long-term" climate sensitivity from paleoclimate data by finding the scale factor that causes the GHG forcing to match the paleoclimate temperature change as accurately as possible. Figure 4 shows that multiplying the climate forcing due to long-lived GHGs (CO2 + CH4 + N2O) by 3.02°C per W/m2 yields remarkably good agreement with Antarctic temperature. Given that glacial-interglacial global temperature change is about half of Antarctic temperature change, this implies a "long-term" climate sensitivity of ~1.5 W/m2 or about 6°C for doubled CO2.

Which climate sensitivity is more relevant to humanity: the Charney 3°C for doubled CO2 or the "long-term' 6°C for doubled CO2? Both. The net human-made climate forcing, including negative forcing by tropospheric aerosols, has been substantially positive only for the past three decades. On that time scale the Charney sensitivity is a good approximation, as little contribution from slow feedbacks would be expected. Thus climate models with 3°C sensitivity for doubled CO2, incorporating only the fast feedbacks, are able to achieve good agreement with observed warming of the past century. We suggest, however, that these models provide only a lower limit on the expected warming on century time scales due to the assumed forcings. The real world will be aiming on the longer run at a warming corresponding to the higher climate sensitivity.

EDIT

http://gristmill.grist.org/story/2007/10/1/131321/083

Link to full Hansen article:

http://www.columbia.edu/%7Ejeh1/East-West_070925.pdf

[Deep13]

1. So it's 2.5 deg. F per deg. C, right? nt

[hatrack]

3. Not quite - an increase of 6C = an increase of 10.8F

Gosh, I sure hope things don't change more quickly than expected!

[Deep13]

5. 11 deg. F is pretty damn bad. nt

[eppur_se_muova]

4. 1.8, or 9/5. nt

[phantom power]

2. Warmer Than Expected

[ramapo]

6. Hansen does have some positive words (a few anyway)

I just read through the whole article. He's one smart guy and he can write a paper on a complex subject that is clear and simple enough for the non-scientist to absorb.

I think his bottom line is that we're in deep shit if we don't address this problem starting today, but if we do, there is time to get things under control. That's the bad news and the good news.

There were two specific points of interest to me. He writes "The net human-made climate forcing, including negative forcing by tropospheric aerosols, has been substantially positive only for the past three decades". So since 1977, when we already knew better, we've been continuing to load up the atmosphere with billions of tons of crap. Funny how that is just about when Reagan's Morning in America travesty got underway which basically ditched most of the environmental progress in favor of unfettered growth and exploitation of resources.

Sadly, this makes you wonder if the current situation might have been avoided had our society gone down a less selfish path. But water over the dam. I guess I need to let that go.

However, since the political, economic and social machine that developed over the past 25 years is firmly in control, it is difficult to see how anything significant occurs in the near-term that will alter our current trajectory. No matter whom is elected in 2008.

Another interesting tidbit is Hansen's assertion that it is likely that the industrial revolution and increased CO2 emissions saved the planet from a long descent into the deep freeze. Some of us remember when scientists warned not of global warming but of eventual widespread cooling. These reports are often mentioned by skeptics who try to disavow global warming by claiming scientists just can't make up their minds.

Actually scientists were spot on according to the data available at the time. Some many billions of tons of CO2 and other gases have changed the climate's direction along the way. That in itself is a pretty sobering thought.