3.3 Modern climate change  (Page 2/17)

Major features in these data include the Medieval Warm Period approximately 1,000 years ago and the Little Ice Age approximately 400 years ago. Even with these events, the bottom panel shows that most of the variability in the NH temperature fits within a 0.5°C temperature range. Rarely has the temperature exceeded the 1961-1990 average, which is the dividing line on this graph. The only major fluctuation outside of this range is during the modern instrument era of the last 300 years, where confidence between the data sets is high. Beginning in the 1800s, the solid black line in each panel traces out approximately a 1°C increase in global temperatures. It is this increase that is the central focus in recent climate change science. Remember from the previous chapter that a 1°C change in the earth’s temperature is a large change; reduce the global average by 4°C to 6°C and much of the NH will be covered with ice as it was 20,000 years ago.

There has been much debate over recent climate change, especially in the news media and among political parties around the world. This debate is centered on the cause of the recent 1°C increase–is it a part of the natural variability in the climate system or have anthropogenic    , which simply means human caused, influences played a major role? In a recent survey given to more than 3,000 college students at the University of Illinois at Urbana-Champaign, it was found the approximately two thirds of those surveyed agreed that recent climate change was due to reasons beyond natural variability in the climate system. (see Figure Recent Climate Change Student Responses ) Approximately 20% reported that the climate change is due to natural changes and the remainder was undecided. Let’s investigate both sides of this argument!

Recall from the Module Milankovitch Cycles and the Climate of the Quaternary that global climate will change as a response to changes in insolation, albedo and the composition of the atmosphere. It was shown that the amount of energy entering the earth-atmosphere system from the sun varies less than 0.1% during the 11-year solar cycle in sunspot activity. Outside of this cycle, the amount of energy from the sun has increased 0.12 Watts per square meter (W/m ² ) since 1750. Is this enough excess energy to produce the 1°C increase in global temperatures that has been observed since the 1800s? As it turns out, the climate system needs nearly 8 times that amount of energy to warm by 1°C. This essentially eliminates fluctuations in solar output as the culprit for recent climate change.

Has the earth’s albedo changed since the 1800s? As we know from the Module Climate Processes; External and Internal Controls , increases in the Earth’s albedo lead to global cooling and decreases lead to warming. The net effect of human existence on Earth is to brighten the surface and increase the global albedo. This change is primarily accomplished through intensive agriculture where forest, marshland, and open prairie are cut down and crops like soybeans, corn, wheat, cotton, and rice are grown in their place. Add this to the current high rates of deforestation in South America and Africa and the evidence is clear that mankind has increased the Earth’s albedo, which should have led to global cooling. (see Figure Deforestation in the Amazon (2010)