Glacier lengths have recently been used to construct a third independent“hockey stick” figure like Figure 10-6, in substantial agreement with the results from treerings and boreholes.Snow accumulate rate tells us something about temperature, because cold air carriesless water vapor, and so snows less. We can also read about temperature in the ratios ofdifferent isotopesin the water. Isotopes of an element have the same numbers ofelectrons and protons as each other, and hence pretty much the same chemistry. That’swhy they are both the same element. The isotopic sisters have different numbers ofneutrons, however, so they have different masses. A water molecule comprised of aheavier isotope of oxygen or hydrogen will tend to rain out earlier than would a watermolecule made of the lighter isotope. As the temperature over an ice field decreases, thewater vapor that remains in the atmosphere to snow down on the ice becomes increasingdepleted in the heavy isotopes in the water vapor. For this reason, there is a systematicrelationship today between the temperature and the relative abundances of the heavy andlight isotopes of oxygen and hydrogen in water, and the temperature at which the snowfell. Isotopic data are also affected by conditions where the water vapor originallyevaporated, and along its trajectory toward the accumulating snow field.What we can conclude from eyeballing the raw data is that climate has varied in thepast millennium, but that there is an ongoing temperature rise in the past centuries unlikeany that has occurred in 1000 years. The next questions are called detection andattributionof anthropogenic climate change. The ironclad method would be to set up
Chapter 106/13/056two Earths, add CO2to one of them, and compare their temperature evolutions. Short ofthat, we can wait for natural experiments like volcanic eruptions, to test our ideas aboutclimate sensitivity. But for CO2proper, the first thing to try is to compare data withmodels. Does rising CO2make sense as an explanation for warming? Is it a betterexplanation than natural variability or natural forcings?Past ForcingsThe first step is to ask about natural processes that drive the climate, natural climateforcings. Different climate forcings can be compared with each other in terms of theirenergy impact on the earth in watts/m2. One W/m2in solar output, for example, isroughly equivalent to one W/m2of infrared light trapped by CO2, or at least these areabout the same in models. We have already discussed the climate sensitivity to doublingCO2, called ΔT2x. A middle-of-the-road estimate for ΔT2xis 3° C. If we wish to thinkabout other forcings than CO2, we can express the climate sensitivity as temperaturechange per W/m2of heat forcing. A typical estimate ΔT/(W/m2) would be 0.75° /(W/m2).Solar intensity varies by 0.2-0.5 W/m2over the 11-year sunspot cycle (Figure 10-7).Solar intensity in the past is estimated by the accumulation rate of the isotope of
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