Climate forcings in the past

The next step is to ask about natural processes that drive the climate, natural climate forcings. Different climate forcings can be compared with each other in terms of their energy impact on the Earth in Watts per square meter. One Watt per square meter in solar output, for example, is roughly equivalent to 1 W/m2 of IR light trapped by CO2, or at least these have about the same impact on model climates.

We can roughly estimate the climate impact of radiative forcings using another version of the climate sensitivity. We have already discussed the climate sensitivity to doubling CO2, called AT2x. A middle-of-the-road estimate for AT2x is 3°C. We can express the climate sensitivity as temperature change per Watts per square meter of heat forcing. A typical estimate AT/(W/m2) would be 0.75°/(W/m2). The current anthropogenic climate forcing today is about 2.5 W/m2 (Fig. 10.10). Multiplying by the climate sensitivity gives us a temperature change estimate of about 1.8 K. This is larger than the Earth has warmed because it takes time for the climate of the Earth to reache equilibrium (Chapter 12).

Solar intensity varies over the 11-year sunspot cycle by about 0.2-0.5 W/m2 (Fig. 11.6). Solar intensity in the past is estimated by the accumulation rate of isotopes that are produced by cosmic rays, such as 10Be and 14C. These are called cosmogenic isotopes. The idea is that a brighter Sun is a better shield to cosmic rays. Cosmic rays produce cosmogenic isotopes. So a high deposition rate of 10Be at some depth in an ice core tells us that the Sun was weaker, unable to deflect cosmic rays, and so we infer that it was not very bright at that time. Who made that story up, you may wonder. We do in fact observe a correlation between cosmic rays, 10Be production, and solar intensity over the 11-year sunspot cycle in recent times (Fig. 11.7). There are times in the past such as the Maunder Minimum, from about 1650 to 1700, when there were no sunspots. This was the coldest period in Europe in the last 1000 years, and glaciers advanced all over the world, including in the southern hemisphere, so it's pretty clear that the solar luminosity was less then than it is now. However, we have no direct solar measurements from a time when the Sun acts like this, so we don't have a very solid constraint on how much lower the solar output was. You can see that there is a factor-of-two uncertainty in the solar forcing in the past, as the thickness of the gray region in Fig. 11.6. The time history of solar forcing variability is to drift up and down on timescales of about 100 years.

Volcanic eruptions inject particles into the atmosphere, sulfate aerosols and dust. Particles injected into the troposphere last a few weeks before they are rained out. It doesn't rain in the stratosphere, so particles spend several years floating around before they eventually sink out. The Mt. Pinatubo eruption in 1991 cooled the planet by 0.5°C for several years, a result consistent with a middle-of-the-road climate sensitivity A T2x of about 3°C, by the way. Volcanic climate forcings look like randomly spaced spikes of random but high intensity in Fig. 11.7.

To the two natural climate forcings we add the two main anthropogenic ones, greenhouse gases and anthropogenic aerosols. Neither was important before about 1750. Greenhouse gases warm whereas sulfate aerosols cool. The greenhouse gas radiative impact is fairly uniform globally whereas the climate forcing from sulfate aerosols is concentrated in the northern hemisphere, downwind from industrialized and heavily populated areas.

Fig. 11.6 Reconstructed history of radiative climate forcings from solar variability, greenhouse gases, anthropogenic aerosols, and volcanic particle emission to the stratosphere. Replottedfrom Crowley (2000).
Fig. 11.7 History of sunspot number and cosmogenic isotope production over the past 1000 years. Data from Beer (2000) and Lean (2000).

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Fig. 11.8 Hadley Centre model simulation of the climate of the last 140 years using (a) natural forcings only, (b) anthropogenic forcings only, and (c) all forcings. Replotted from IPCC (2001).

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Fig. 11.8 Hadley Centre model simulation of the climate of the last 140 years using (a) natural forcings only, (b) anthropogenic forcings only, and (c) all forcings. Replotted from IPCC (2001).

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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