The nation’s top climate scientist is very prolific: He has co-authored another important article: “Global Warming: East-West Connections.” And I’m not just saying that because he cites one of my articles. In fact, we’ve been having an e-mail exchange, and he strongly disagrees with me that it is too late in a practical sense to save the Arctic (and hence the polar bear). He believes strong and smart action now could work — whereas I believe we need such action now to save the Greenland ice sheet, but doubt we can or will act in time to stop the total loss of Arctic summer ice.
I have previously written about the crucial climate variable — the equilibrium climate sensitivity (typically estimated at about 3°C for double CO2) — and how it only includes fast feedbacks, such as water vapor. Now Hansen has a draft article that looks at both current climate forcings and the paleoclimate record to conclude that ‘long-term’ sensitivity is a stunning 6°C for doubled CO2. Here is what Hansen says on the subject (though when you read it you may wonder why Hansen is more optimistic than I am, rather than less):
The implied climate sensitivity, ¾°C per W/m2 (equivalent to 3°C for doubled CO2), is consistent with climate model estimates, but of greater precision. We can never be certain that climate models accurately include all relevant processes. But we know that the real world included all changes of clouds, water vapor, sea ice and any other such fast ‘feedbacks’ that exist.
It must be recognized that the specific climate sensitivity derived in this way includes only ‘fast’ feedbacks. We call this the Charney climate sensitivity, because it is essentially the case considered by Charney (1979), in which water vapor, clouds and sea ice were allowed to change in response to climate change, but GHG (greenhouse gas) amounts, ice sheet area, sea level and vegetation distributions were taken as specified boundary conditions. We would expect the Charney climate sensitivity to be most relevant on decadal time scales. On longer time scales as the quantities assumed to be fixed can change in response to climate change, thus becoming powerful climate feedback mechanisms.
Important insight emerges from close examination of the temperature and forcing curves: the temperature change leads the forcings by several hundred years. Thus the greenhouse gas and ice sheet changes, although they are the principal direct mechanisms for the climate change, are changing as feedbacks. The pacemaker and instigator of the changes is cyclic variation of the Earth’s orbit (Hays et al. 1976), which alters the seasonal and geographical distribution of solar radiation. Insolation changes by a significant amount over several thousand years.
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.
Note that the 6°C sensitivity for doubled CO2 applies to the Pleistocene. About half of that sensitivity is from the ice sheet albedo feedback. At earlier times in the Cenozoic, between 65 and 35 My BP when there was little ice on the planet, the sensitivity should have been closer to the Charney 3°C sensitivity.
Elsewhere (Hansen et al. 2007a) we have described evidence that slower feedbacks, such as poleward expansion of forests, darkening and shrinking of ice sheets, and release of methane from melting tundra, are likely to be significant on decade-century time scales. This realization increases the urgency of estimating the level of climate change that would have dangerous consequences for humanity and other creatures on the planet, and the urgency of defining a realistic path that could avoid these dangerous consequence.
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Joe,
The focus now is on solutions. From a policy point of view, the science is “settled” and it is not necessary to convince the unconvinced to go forward. Heck, I’m not convinced even though I support your efforts and your goals. What would be your target for CO2 in 2050? I imagine it would be even lower than the new British standard.
Where are we now nationally and world wide? Is there information available about each country’s CO2 emissions over the years? Are ours going up, down or holding steady?
Paul K — The U.S. contribution of carbon to theactive carbon cycle continues to grow with the economy and population, despite a somewhat greater energy efficiency.
The world as a whole adds about 8 billion tonnes (Gt) of carbon to the active carbon cycle each year currently, up from about 7 Gt a few year ago.
There is a total excess of about 500 Gt of carbon in the active carbon cycle. This is approximately half caused by the U.S. economy and half by Europe’s.
Try the Carbon Dioxide Information Analysis Center’s excellent web site for greater detail.
Paul: I agree that the science is settled — but I still think it is important to stay up to date on the best writing and research.
I hope to do a multi-part series on the solution soon — after the website gets redesigned.
Global emissions have been soaring. China and the U.S. account for most of the projected growth in the near future — we hold the future in our hands. I’d like to see a 60% to 80% reduction by midcentury, but the key is to get started as quickly as possible off of business-as-usual.
Thanks for the quotations, but do you have a link for the article?
Oh I see: you mean the article you referred to in the previous paragraph: Global Warming: East-West Connections. This wasn’t clear to me.
It’s nice to see that Hansen is aknowledging the Sun’s role in all of this, even if many pundits and bloggers still insist it’s a myth.
And here’s a new article about it -
http://www.environmentnc.com/?p=272
there is not and will not be a serious effort to control or reduce carbon emissions: the world is headed for a cataclysm and nothing will stop it. we will continue to dump carbon into the atmosphere until the ecosystem collapses and finally we as a species are severely reduced if not eliminated.
Before people write much more about this subject, they should go to the CarbonEquity.info website and download ‘The Big Melt – Lessons from the Arctic summer of 2007.’ The graph on page five is a real shocker. When I show it to others, they turn pale.
would you be kind enough to check on the address for carbon.equity.info..i tried looking for it on yahoo and nothing…
It can be found at :
http://www.carbonequity.info/
The unanswered question is: what should we do and when do we start? We know what needs to be done but we know it hasn’t started, notwithstanding the scam called carbon trading or the feeble legislation that recently passed congress. Why is Jim Hansen the only scientist making his voice heard in the public and political process? Why isn’t the scientific community backing him up with petitions, advertisements in publications, public lectures, radio and TV interviews, direct mailings, and, above all,a strong uncompromising lobbying presence in the halls of congress demanding tough reductions in energy use in the next ten years? Why are they not coming out in opposition to coal burning power plants, both operating and planned? Why are they not opposing fossil fuel subsidies? Why are they not publicizing the false hopes of carbon sequestration? They should be among the strongest environmental advocates. Jim Hansen deserves huge credit for what he is doing but this doesn’t get the other scientists off the hook. They can’t use their professional expertise to hide from their civic responsibility as CITIZENS to speak out and act. You don’t give up your political rights when get a science degree or job at a university or research lab. They should be organizing a Million Scientist March on Washington to demand energy policies commensurate with the dangers and imminence of climate change.
if you apply the same long term effects to solar output that we know happened is there any temperature change left to have to explain?
Steve, over tens of millions of years the major trend (ignoring the smaller 100,000 year variations) has been the sun’s output has increased while temperature has slowly fallen as CO2 has fallen. The exceptions have been events in Earth’s history, such as the Paleocene-Eocene Thermal Maximum, where volcanoes and other sources added greenhouse gases to the atmosphere, and things warmed dramatically for a 100,000 years, and then CO2 and temperature both resumed their downward trend. So one concludes that temperature responds to CO2. There is a response to solar insolation which is well understood, but CO2 is capable of overwhelming that process, as shown by Earth’s history.