What is global temperature?

When I write here about climate change you can be sure one or more trolls will pop up and tell me about the snow in the UK. Or accuse scientists of relying on this year’s New Zealand drought in their claims of climate change. It seems surprising that no matter how many times we point out the climate is not the same as weather, and global climate change is not the same as local or regional weather, some people seem to think such arguments are valid.

I have illustrated the difference before here – but here’s another example I picked up from Real Climate (see Response by Marcott et al.). The illustration is in response to the question of what is meant by global Temperature. Here’s what they say:

“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.”

ProxySites_vs_Global1-1024x907

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).

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One response to “What is global temperature?

  1. Pingback: Another Week in the Planetary Crisis, April 7, 2013 – A Few Things Ill Considered

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