There is a cooling; there is a heating.
What an elegantly simple way of describing – not climate. Perhaps the seasons. Temperatures generally rise in spring and summer and decline in fall and winter. But these seasonal changes are not climate. If one is to talk about climate, one should use the word correctly. By definition, “climate” is a description of long-term trends in weather. Trends that are at least ten years long, often more. Climate is not an instantaneous measure of temperature, but includes a long-term average.
By those rules, there has been a recent heating of Earth. Every year for the past 17, the average global temperature has been at least 0.4° C warmer than the average during the 20th century. That’s clearly “a heating.” By those rules, the average global temperature on Earth has increased by about 0.8°C since 1880. On the other hand, there hasn’t been a cooler-than-average year since 1976. It would be more correct to say, “Forty-one years ago, there was a cooling; since then, there has only been a heating.”
Eight-tenths of a degree C? That’s just a shade less than one-and-a-half degrees F. Difficult, but not impossible to detect with a tongue or a fingertip. On the other hand, that is an average of temperature readings taken from around the world. In some places, the local average increase has been higher than 0.8°C. In some places, the local average increase has been lower. (Ref 1)
Earth is warmed by the sun, by radioactive decay from deep within Earth, and by heat left over from the primordial Earth (the Hadean Eon, from about 4.5-to-4.0 billion years ago). What is important about the last two is that they are slowly but inexorably decreasing. (Ref 2) The only source of an increase in Earth’s average temperature is the sun.
The sun’s output is known to fluctuate. For example, there is an eleven-year cycle in the number of sunspots.
When there are more sunspots, the sun radiates more light than when there are fewer. In addition, when there are fewer sunspots, there is a lighter solar wind. More cosmic rays strike Earth’s atmosphere and may create aerosol particles that may create more clouds which reflect more sunlight. (Ref 5)
Recent research (Refs 3, 4) offers other measures of solar activity, and suggests an increase in some measures during the early 20th century. The effect on climate, if any, is not yet known; however, solar activity has leveled off in the past several decades, while average temperatures on Earth have continued to rise.
The amount of energy from the sun that is absorbed by Earth depends, first, on a number of cyclical factors, including the eccentricity of orbit (how close to the sun the earth is) and some wobbling of Earth’s axis, as well as the imbalance in land area in the northern and southern hemispheres. These factors were first captured by Milutin Milancović in the 1920s. Please see Ref 6 to begin a look at this.
We should be able to agree, at this point, that Earth’s internal heat is not causing a rise in average global temperatures, but that the sun’s affect on Earth’s average global temperature is the culprit (if culprit is the right word).
There is not, however, enough of a difference in solar energy explained by either the Milancović cycles or sunspot cycles to account for the recent (since 1880) increase in average global temperatures. A simple reason is that these things are, after all, cyclical and we would expect they would “level out” over the past nearly 140 years.
If you will accept what has been presented so far, perhaps you would also be willing to accept the notion that the rise in average global temperature can be laid at the feet of greenhouse gasses.
Registered Curmudgeon, scientist, skeptic, humanist, and writer.