How much does the global climate system vary naturally?
Trying to separate out the human impact on climate from the natural chaotic signal is a major scientific challenge. The same effect (increasing temperatures) can arise from multiple causes. Therefore, jumping to conclusions in climate research can be a risky business. If one were to use temperature records from the city of London, one might conclude that the planet has indeed warmed—until it would be pointed out to you that your record merely reflected the growth of the London heat island, a local effect.
It is known that climate has always fluctuated wildly. There is no reason to suspect it has suddenly stopped doing this as a result of natural processes. Some of the bits of evidence that scientists are sorting through include the following items.
By best estimates, the average temperature of the Earth is getting warmer. Scientists estimate that the planet’s temperature has risen about 0.6°C during the past 100 years. But the cause is open to more debate since this change is well within the suspected normal fluctuations of the atmosphere’s temperature. While the accumulation of greenhouse gases is certainly a favored theory, other natural factors such as changes in the output of the sun cannot yet be ruled out as at least a partial contributor. A period of increasing temperatures by itself cannot be taken as conclusive proof of human-induced global warming.
The period from 800 to 1250 in Europe is sometimes called the Little Climatic Optimum. The moderate weather was a main reason for the wave of Norse settlers heading for Iceland, Greenland, and even North America, as the mild climate allowed not only for livestock grazing, but also the growing of some crops. Vineyards even thrived in England.
A sudden planetary cooling has become known as Europe’s Little Ice Age. It was most intense in the seventeenth century, during which the Thames River in London froze over solid at least ten times. The Viking expansion from Scandinavia into Ice land, Greenland, and Newfoundland during the early medieval period was abruptly terminated around 1400 at the start of the Little Ice Age, which forced the abandonment of many of these northerly outposts.
Attempting to assess whether or not human enhancement of the “greenhouse effect” is causing global warming is tricky business, if only because other influences are constantly acting upon the Earth’s mean temperature. For instance, the El Niño of 1986—87 caused a global warming of as much as 0.6°C, while the eruption of Mt. Pinatubo caused a cooling, also of about 0.6°C.
The burning of fossil fuels has raised the carbon dioxide levels in the Earth’s atmosphere to their highest levels in 160,000 years. But what caused, and later ended, an earlier, high-CO event is unknown.
The planet’s delicate thermal balance has been upset without the polluting intervention of humankind. The national Ocean Drilling Program has analyzed bottom sediment cores and found that around 57 million years ago, at the end of the Paleocene epoch, sea temperatures rose from 54°F to 64°F. This resulted in the disappearance of up to 50 percent of the species inhabiting the deep seas.