Figure 1 shows how Earth’s average temperature has changed over time from two research groups â€“ one the National Climate Data Center in the United States, and the second from the Climate Research Unit at the University of East Anglia in the United Kingdom.
Figure 1. Thermometer-based global average temperature as a function of time, plotted relative to the average between 1960 and 1990. This includes land and ocean. From the NOAA Web page.
In both graphs, temperatures are compared to the 1960-1990 averages. Any temperature below the red “zero” line is cooler than that period, and any temperature above the red “zero” line is warmer. The temperature records are carefully chosen to avoid too much influence of the warming of cities (see previous blogs on the urban heat island), changes in thermometers at a given site, changes of measurement locations, and other changes that influence local temperature. And, because oceans cover 70% of Earth’s surface, extra care is taken in analyzing sea surface temperature data from ships. You can find more information about how temperature data are treated in a 1990 Scientific American article by Philip D. Jones and Tom M.L. Wigley (pp 84-91). A graph of surface air temperature relative to the 1951-1980 mean, from the Goddard Institute for Space Studies (GISS), shows similar trends (Figure 2). In this case, satellite-based sea-surface temperatures are used from 1982 on.
The graphs are for Earth’s yearly average temperature at about 1.5 m above the surface (the height temperatures are measured by weather services, and the height at which GLOBE students take their temperature observations, see Instrument Construction from the Teacher’s Guide, p 14). That includes the land, which covers 30% of the surface, and the ocean, which covers 70% of the surface.
The GISS graph in Figure 2 includes the temperature departures from the 1951-1980 averages. You can see that some places â€“ like the Arctic, are much warmer than that average. And some paces, like parts of the high latitudes in the Southern Hemisphere, are actually cooler.
Figure 2. (Top) temperature departure from 1951-1980 mean as a function of time. (Bottom) map of temperature departure from 1951-1980 mean for 2005. Source: Goddard Institute for Space Studies. Numbers are degrees Celsius.
We tend not to remember averages, though. We remember hot days, or cold days, or storms, so we (even scientists!) are tempted to think that a hot day means “global warming” and a record low means that “global warming” isn’t happening. That is why looking at graphs of average temperature data is helpful. The graph enables us to study the patterns in climate data which represent average weather over time.
What is the difference between weather and climate? In the words of a middle-school student, “Weather helps you decide what to wear, and climate helps you decide what clothes to buy.”
Next: What affects Earth’s climate?