An introduction to a complex field
By Eric J. Lerner, LPPFusion Chief Scientist
Some things about global climate change are clear, with good scientific evidence, but much is not yet known, mainly because the earth’s climate system is so complex.
There is good evidence that since about 1975, the earth has warmed up on average around 0.6 °C (1 degree F). This is certainly comparable too, and, depending on the data series, possibly larger than the biggest short term fluctuations in climate that have occurred over the past several thousand years, a period of relative climatic stability after the end of the last Ice Age. By comparison, during the last Ice Age global temperature was about 8-10 °C lower than at present.
Similarly there is strong evidence that sea level has risen about 10 cm over the same 40-year period. Again this seems to be larger than historical variations over at least the past 1,000 years. But it is tiny compared with the 150 meter rise that came with the end of the Ice Age.
The changes in the Arctic have been far more dramatic than global changes, with the summer extent of Arctic Ocean sea ice dropping rapidly by about 50%, starting in the late 1990’s. (Since sea ice floats, its melting does not contribute to sea level change.)
Global weather has also gotten sunnier, with clear skies increasing by about 10% since 1985. This alone could account for the warming trend, but it could as well be a consequence of it.
The longer-term trends, which include a warming from the late 1880’s to around 1950, are complicated by the fact that the global climate was considerably cooler in the 18th century the the average for the past thousand years, so part or all of that early-20th century warming was due to a return to more normal temperatures.
When it comes to causes, there are several possibilities. Carbon dioxide increases from fossil fuels is certainly the most popular one, but temperature increases do not closely track CO2 increases in recent decades, and in ancient data temperature increases may precede CO2 increases, according to some researchers.
A second human contribution to global warming is destruction of the tropical rain forest. This deforestation accelerated greatly around 1970 for economic reasons and led to the reduction of rainforest areas by about 30%. Since rainforests recycle moisture into the atmosphere, creating low-lying clouds, deforestation could have contributed significantly to the reduction in cloud cover and thus to the global temperature rise.
A third human warming effect is caused high-altitude aerosols, generated by airliners. These aerosols can be effective in preventing infrared radiation from escaping into space. Fourth, soot from fossil fuel burning can settle on snow or ice-covered regions, making them darker and allowing them to absorb more solar radiation. In turn, more rapid melting of ice and snow, due in part to this soot darkening, uncovers much darker underlying land or ocean, increasing the warming effect.
So, there are many human contributions to global warming beyond CO2.
In addition there may be natural contributions to warming. Some data points to a decades-long increase in solar radiation, although this data is subject to errors, as it was collected by several different satellites, each with its own calibrations. And there are longer term variations that are not fully understood, like that which caused a cold 18th century and a much warmer late Middle Ages.