Response by Marcott et al.

A: Global average surface temperature is perhaps the single most representative measure of a planet’s climate since it reflects how much heat is at the planet’s surface. Local temperature changes can differ markedly from the global average. One reason for this is that heat moves around with the winds and ocean currents, warming one region while cooling another, but these regional effects might not cause a significant change in the global average temperature. A second reason is that local feedbacks, such as changes in snow or vegetation cover that affect how a region reflects or absorbs sunlight, can cause large local temperature changes that are not mirrored in the global average. We therefore cannot rely on any single location as being representative of global temperature change. This is why our study includes data from around the world.

We can illustrate this concept with temperature anomaly data based on instrumental records for the past 130 years from the National Climatic Data Center (http://www.ncdc.noaa.gov/cmb-faq/anomalies.php#anomalies). Over this time interval, an increase in the global average temperature is documented by thermometer records, rising sea levels, retreating glaciers, and increasing ocean heat content, among other indicators. Yet if we plot temperature anomaly data since 1880 at the same locations as the 73 sites used in our paleotemperature study, we see that the data are scattered and the trend is unclear. When these same 73 historical temperature records are averaged together, we see a clear warming signal that is very similar to the global average documented from many more sites (Figure 1). Averaging reduces local noise and provides a clearer perspective on global climate.

Figure 1: Temperature anomaly data (thin colored lines) at the same locations as the 73 paleotemperature records used in Marcott et al. (2013), the average of these 73 temperature anomaly series (bold black line), and the global average temperature from the National Climatic Data Center blended land and ocean dataset (bold red line) (data from Smith et al., 2008).

New Scientist magazine has an “app” that allows one to point-and-plot instrumental temperatures for any spot on the map to see how local temperature changes compare to the global average over the past century (http://warmingworld.newscientistapps.com/).

Q: How does one go about reconstructing temperatures in the past?

A: Changes in Earth’s temperature for the last ~160 years are determined from instrumental data, such as thermometers on the ground or, for more recent times, satellites looking down from space. Beyond about 160 years ago, we must turn to other methods that indirectly record temperature (called “proxies”) for reconstructing past temperatures. For example, tree rings, calibrated to temperature over the instrumental era, provide one way of determining temperatures in the past, but few trees extend beyond the past few centuries or millennia. To develop a longer record, we used primarily marine and terrestrial fossils, biomolecules, or isotopes that were recovered from ocean and lake sediments and ice cores. All of these proxies have been independently calibrated to provide reliable estimates of temperature.

Q: Did you collect and measure the ocean and land temperature data from all 73 sites?

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