When I arrived at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, in the 1970s, I started hearing the debate about whether the global climate was getting warmer or cooler. From the “warming” graph in the previous blog, there wasn’t much of a trend at that time.

On our bulletin board, we posted two articles next to each other. One was about an NCAR scientist who said that Earth’s climate was getting warmer because of more carbon dioxide in the atmosphere. The second was about a scientist who said it was getting cooler because of changes in Earth’s orbit and tilt. The changes, he argued, could mean the beginning of a new Ice Age in a few hundreds of years or so. Beneath the two articles, we posted Robert Frost’s Poem “Fire and Ice,” because it starts as

“Some say the world will end in fire,
Some say in ice.”

Surrounded by talk about climate, I retreated into the library and found a wonderful book that was published in the early 1900s, and I realized I hadn’t heard much new about the factors that affect Earth’s climate – the ingredients were:

• How much sunlight is available to heat Earth
• How much heat escapes to space

Which are related to:

• What gases are in the atmosphere (especially carbon dioxide and water vapor)
• How much dust there is in the atmosphere (including that resulting from volcanoes)
• How much ice there is on Earth’s surface (since ice reflects light back to space)
• Where the continents are

And, as I mentioned earlier under “regional climate”

• Properties of the rest of Earth’s surface (other than ice cover)

And of course stuff going on in the atmosphere:

• Amount and height of clouds
• Winds, etc.

And

• What the ocean is doing

Finally, Walter Orr Roberts, the head of NCAR at the time, was interested in the effects of:

• The changing sun

And, as noted in the article about the scientist who thought an ice age was coming:

• Changes in Earth’s orbit and tilt

Figure 1 shows how Earth’s average temperature has changed over time from two research groups – 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 – 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?