Palaeoclimate data show that the Earth's climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the ‘albedo flip’ property of ice/water, provides a powerful trigger mechanism. A climate forcing that ‘flips’ the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Inertia of ice sheet and ocean provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are also important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years. The most important of the non-CO2 forcings is methane (CH4), as it causes the second largest human-made GHG climate forcing and is the principal cause of increased tropospheric ozone (O3), which is the third largest GHG forcing. Nitrous oxide (N2O) should also be a focus of climate mitigation efforts. Black carbon (‘black soot’) has a high global warming potential (approx. 2000, 500 and 200 for 20, 100 and 500 years, respectively) and deserves greater attention. Some forcings are especially effective at high latitudes, so concerted efforts to reduce their emissions could preserve Arctic ice, while also having major benefits for human health, agricultural productivity and the global environment.



  • One contribution of 18 to a Discussion Meeting Issue ‘Trace gas biogeochemistry and global change’.

  • We include climate-driven aerosol changes and their cloud effects as a ‘fast feedback’ because aerosols respond rapidly to climate change. This choice yields a more precise empirical climate sensitivity because aerosol forcing depends sensitively on uncertain aerosol absorption. Our inferred climate sensitivity, 3°C for doubled CO2, is the same as estimated by Hansen et al. (1993), who did not classify aerosols as a fast feedback, because our present omission of the small net aerosol forcing is compensated by larger effective GHG forcings, especially the high efficacy (140%) of CH4. Ice core data show that aerosols decrease as the climate warms, probably because increased water vapour and rainfall wash out aerosols. Aerosol amount in the Earth's atmosphere seems to have decreased in the past two decades (Mishchenko et al. 2007), while human-made aerosol sources were believed to be increasing. We suggest that the aerosol decrease may be due to rapid global warming, approximately 0.2°C per decade (Hansen et al. 2006a), and resulting moistening of the atmosphere.

  • Antarctic temperature change divided by 2 serves as a crude ‘global thermometer’ for large global climate change on time-scales of several thousand years or longer. Limitations of a local thermometer are obvious on time-scales of 1–2 kyr or less, when Antarctic and Greenland temperature fluctuations are often on a ‘see-saw’, i.e. out of phase (EPICA 2006). Leads and lags of temperature changes at different locations are crucial for understanding the mechanisms of climate change, and these short-term variations can involve complex dynamical processes, including possible ‘reorganizations’ of ocean and atmospheric circulation. However, global temperature changes must be coherent in the two hemispheres for any climate forcings large enough to change tropical ocean temperature, because the tropics, via ocean and atmosphere, export heat to both hemispheres. Indeed, a coherent global response occurs even for forcings predominately located in one hemisphere, such as anthropogenic aerosols or change of ice sheet area, although the response is larger in the hemisphere with greater forcing (Hansen et al. 2005a).

  • The last glacial cycle, the most accurately dated, has two notable discrepancies between observed and calculated temperature. The first calculation discrepancy is failure to obtain a deep minimum temperature at Marine Isotope Stage (MIS) 5d (ca 110 kyr BP). We suggest that the sea-level curve of Siddall et al. (2003) may understate sea-level fall at that time. Several other sea-level records, summarized in fig. 6 of Potter & Lambeck (2003), show sea level much lower in MIS 5d than in MIS 5c (ca 105 kyr BP). The second discrepancy occurs in the last 8000 years, with calculated temperature rising rapidly while observed temperature fell. Calculated warming is due to increase of CO2 from approximately 260 to 275 ppm and CH4 from approximately 600 to 675 ppb. Ruddiman (2003) suggests that the GHG increases are due to deforestation beginning ca 8 kyr BP and rice agriculture beginning ca 5 kyr BP. Indeed, much of the observed GHG increase is plausibly anthropogenic, but we would expect early negative anthropogenic forcings from the same agricultural and deforestation activities, due to aerosols and surface albedo change (not included in figure 2), to exceed positive GHG forcings. The aerosol forcing, especially indirect effects on clouds, is strongly nonlinear, with human-made aerosols in a nearly pristine atmosphere being much more effective than those added to the present atmosphere. Thus, although Ruddiman's basic thesis is probably correct, his conclusion that humans saved the Earth from an ice age is probably not right.

  • Climate sensitivity, including the fast-feedback sensitivity, changes with the mean state of the Earth's climate (Hansen et al. 2005a). However, climate sensitivity is practically independent of time over the past several hundred thousand years (figure 2).

  • The potential of these ‘amber waves of grain’ and coastal facilities for permanent underground storage ‘from sea to shining sea’ to help restore America's technical prowess, moral authority and prestige, for the sake of our children and grandchildren, in the course of helping to solve the climate problem, has not escaped our attention.

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