How can we avert dangerous climate change

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Climate is an average of weather over some period, including the variability and extremes within that period. Because day-to-day weather fluctuations are so large, it is not easy to notice small changes of the average weather or climate.

Climate is an average of weather over some period, including the variability and extremes within that period. Because day-to-day weather fluctuations are so large, it is not easy to notice small changes of the average weather or climate. However, moderate changes of climate can have significant effects, for example, on the ability of plants and animals to survive in a given region and on the stability of large ice masses and thus sea level.

Climate varies a lot without any help from humans. In part the variations are simply chaotic fluctuations of a complex dynamical system, as the atmosphere and ocean are always sloshing about. The climate also responds to natural forcings, such as changes of the brightness of the sun or eruptions of large volcanoes, which discharge small particles into the upper atmosphere where they reflect sunlight and cool the Earth.

Climate forcing. A climate forcing is a perturbation of the Earth’s energy balance that tends to alter the Earth’s temperature (Hansen et al. 2005a). For example, if the sun’s brightness increases 2% that is a positive forcing of about 4.5 W/m2 (watts per square meter), as it results in an increase of that amount in the energy absorbed by the Earth. […] The Earth responds to this forcing by warming up until its thermal radiation to space equals the energy absorbed from the sun.

Doubling the amount of carbon dioxide (CO2) in the atmosphere causes a global climate forcing similar in magnitude to that for a 2% increase of solar irradiance. The CO2 forcing works by making the atmosphere more opaque to infrared radiation, the wavelengths of the Earth’s heat radiation. […] [The] energy radiated to space with doubled CO2 is reduced by an amount that is readily calculated from radiation physics to be approximately 4 W/m2. So the planet’s energy imbalance is about the same as for a 2% increase of solar irradiance. In either case, the Earth responds by warming up enough to restore energy balance. […]

Climate sensitivity and climate feedbacks. Global climate sensitivity is usually defined as the global temperature change that occurs at ‘equilibrium’, i.e., after the climate system has had a long time to adjust, in response to a specified forcing. The specified forcing is commonly taken to be doubled CO2, thus a forcing of about 4 W/m2.

Climate sensitivity can be evaluated either theoretically, with the help of climate models, or empirically, from the Earth’s climate history. In either case, it must be recognized that the climate sensitivity so inferred depends upon what climate variables are fixed as opposed to being allowed to change in response to the climate forcing. […]

In reality all of [the] boundary conditions can change in response to climate change, becoming either positive climate feedbacks (amplifying the climate change) or negative feedbacks (diminishing the climate change). The choice of feedbacks that were allowed to operate in the Charney (1979) study (water vapour, clouds, sea ice) was in part based on realization that these variables change rapidly, i.e., they are ‘fast feedbacks’. Thus if one is interested in climate change on the time scale of decades or longer, these feedbacks must be allowed to operate. Ice sheets and forest cover, on the other hand, might be considered ‘slow feedbacks’, not expected to change much on decadal time scales. […]

The Charney (1979) study suggested that equilibrium climate sensitivity was ~3°C (5.4°F) for doubled CO2, with uncertainty at least 50% (1.5°C). Improving climate models continue to yield global climate sensitivity ~3°C for doubled CO2, but uncertainty remains because of the difficulty of accurately simulating clouds. […]

Climate response time. A practical difficulty with climate change arises from the fact that the climate system does not respond immediately to climate forcings. […] The climate response to a forcing introduced at time t = 0 […] requires about 30 years for 50% of the eventual (equilibrium) global warming to be achieved, about 250 years for 75% of the response, and perhaps a millennium for 90% of the surface response.

The exact shape of this response function depends upon the rate of mixing in the ocean, thus upon the realism of the ocean model that is used for its calculation. The response time also depends upon climate sensitivity, the response being slower for higher sensitivity. The reason for this slower response is that climate feedbacks come into play in response to climate change, not in response to the forcing per se, and thus with stronger feedbacks and higher climate sensitivity the response time is longer, indeed, it varies with the square of climate sensitivity (Hansen et al. 1985). The curve […] was calculated for sensitivity 3°C for doubled CO2.

This long response time means that even when GHGs stop increasing, there will be additional warming “in the pipeline”. Thus we have not yet felt the full climate impact of the gases that have already been added to the atmosphere. This lag effect makes mitigation strategies more arduous.

Slow climate feedbacks. The ‘Charney’, or fast feedback, climate sensitivity is intended to be relevant to decadal time scales. But it is becoming clear that other feedbacks, omitted because they are ‘slow’ and difficult to deal with, may also be important.

One ‘slow’ feedback is the poleward movement of forests with global warming. If evergreen forests replace tundra and scrubland vegetation, it makes the surface much darker. Trees are ‘designed’ to capture photosynthetic radiation efficiently, and thus they can provide a strong positive climate feedback. Forest cover is a powerful positive feedback at Northern Hemisphere high latitudes, and significant changes are already beginning (Zhou et al. 2001; Piao et al. 2006). Although this positive feedback may be partially balanced globally by higher subtropical surface albedo due to increasing desertification, the positive feedback dominates in the regions of possible sea ice and ice sheet tipping points.

Another ‘slow’ feedback is associated with ice sheets. An ice sheet does not need to disappear for significant feedback to occur: just the change of ice surface albedo (reflectivity) that occurs with increased melt area and melt season duration contributes a large local climate feedback. […] Increased areas of surface melt, and lengthening melt season, are observed on both Greenland (Steffen et al. 2004; Fettweis et al. 2007; Tedesco 2007) and West Antarctica (Nghiem et al. 2007).

Still another ‘slow’ feedback is the effect of warming on emissions of long-lived GHGs from the land or ocean. Melting of tundra in North America and Eurasia is observed to be causing increased ebullition of methane from methane hydrates (Archer 2007; Zimov et al. 2006). In addition, the ability of the ocean to absorb human-made CO2 decreases as the emissions increase [Archer, 2005], and there is a possibility that the terrestrial biosphere could even become a source of CO2 [Cox et al. 2000; Jones et al. 2006]

It is apparent that at least some of these ‘slow’ feedbacks, which are primary causes of the very high climate sensitivity on paleoclimate time scales, as discussed below, are beginning to operate already in response to the strong global warming trend of the past three decades.

How can we avert dangerous climate change

James HANSEN

Select Committee on Energy Independence and Global Warming, United States House of Representatives

April 26th 2007

http://globalwarming.house.gov/

http://www.columbia.edu/~jeh1/canweavert.pdf

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