The climate of the last millennium

We would like to extend the temperature record further back in time, before the era of technological measurements. The thermometer and satellite temperature records indicate a warming trend since about 1970, but this begs the question: is this warming normal, or is it something new? Temperatures from before the days of thermometers can be estimated by a variety of tricks, measurements of something we can measure, interpreted as indicators of things we would like to know. These alternative measurements are known as proxy measurements.

One source of proxy temperature measurements is tree ring thickness (Fig. 11.9). In places where winter is much colder than summer, wood that grows in winter is a different color than wood from summer, leading to the formation of annual rings. Trees

Year AD

Fig. 11.9 Northern hemisphere mean temperature reconstructions. Tree rings from IPCC (2001), boreholes from Rutherford and Mann (2003), and glacier length from Oerlemanns (2005).

Year AD

Fig. 11.9 Northern hemisphere mean temperature reconstructions. Tree rings from IPCC (2001), boreholes from Rutherford and Mann (2003), and glacier length from Oerlemanns (2005).

grow more quickly during warmer years than colder years, so it makes sense that tree ring thickness might carry information about temperature. Long records of tree ring thicknesses can be assembled from collections of many individual trees that lived long ago, by matching the patterns of widths to fit the different trees together in time. The widths of the rings must then be calibrated against temperature changes from part of the record where the temperature is known independently. This is tricky because trees grow or don't grow in response to other environmental variables, such as the availability of water, nutrients, and even atmospheric CO2. Young trees grow more vigorously than old ones. Temperatures vary from place to place, and so calculating a global average from local data introduces another possibility of bias.

Other proxy measurements for temperature include the temperature as a function of depth in the Earth and in ice cores, called borehole temperatures. The temperature goes up with depth in the Earth or in the ice because heat is being conducted from the interior of the Earth to the surface. The steeper the temperature change with depth, the more heat is transported. If we imagine holding the temperature at the Earth's surface constant for a long time, then the temperature profile with depth in the Earth or the ice would be a straight, linear increase with depth (Fig. 11.10). If we now imagine warming the surface of the Earth, then the warming will propagate down into the Earth or the ice. If we measure the temperature profile today, we can use mathematical methods called inverse methods to figure out what the surface temperature was in the past, in order to generate what we see today. Borehole temperature records lose the little wiggles in the real temperature record, and memory fades with time so the uncertainty gets greater. Borehole temperatures show warming throughout the past 500 years, more or less consistent with the tree ring record (Fig. 11.9).

Other sources of information about past climates come from ice sheets and mountain glaciers. Glacier lengths have recently been used to construct a third independent


Variations at Earth surface

Depth in Earth or ice

Background geothermal temperature gradient

Background geothermal temperature gradient u

Fig. 11.10 Time evolution of subsurface temperature, when the surface warms up. From a snapshot of the current temperature profile (black line), scientists can reconstruct the history of the surface warming (arrows).

prehistoric temperature reconstruction in Fig. 11.9, in substantial agreement with the results from tree rings, boreholes, and where it exists, the instrumental temperature record.

Temperature reconstructions of the last millennium sometimes show a period of warmth from 800 to 1200, called the Medieval Warm period, and cooler temperatures from 1500 to 1800, called the Little Ice Age. Tree ring records mostly come from the northern hemisphere, so it is not clear whether the Little Ice Age and Medieval Warm periods were global or not. The timings of warmings and coolings, where they are documented in the northern hemisphere, do not seem to be precisely synchronous.

Models are able to simulate the Medieval Warm and Little Ice Age, largely as a result of long-term changes in solar forcing. The little ice age in particular coincided with a time of low solar flux inferred from cosmogenic isotope production (Fig. 10.6), and the Maunder Minimum dearth of sunspots (Fig. 10.7). The climate record of the past millennium is a less stringent test of the climate models than the record from the last century, though, because there is more uncertainty in both the radiative forcing and in the temperature record we are trying to match. However, we can draw the striking conclusion from the millennial climate record that the warming of the twentieth century, especially in the last few decades, stands out as something new that has not happened before. The warming we observe in the past decades is more intense, faster, and more obviously global than the Medieval Warm or Little Ice Age periods in the past.

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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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