Hello Weather Underground bloggers, Aaron here. Dr. Masters is on vacation so I'll be posting a series of vacation blogs for him. This is the first.
In my previous blog, I mentioned how the Arctic Climate Impact Assessment
(ACIA), a 2004 study of Arctic climate compiled by 300 scientists over three years, found that the recent increase in Arctic temperatures was probably due to human-emitted greenhouse gases. Greenhouse skeptics attacked the ACIA report and its conclusion, pointing out that the Arctic was much warmer than today during the period 4,000 - 7,000 years ago. For example, Dr. Patrick Michaels said in a 2004 interview with CNS news
, a conservative Internet news service:"It was warmer 4 to 7,000 years ago [in the Arctic.] Every climatologist knows that. I saw no mention of that in the Arctic report," Michaels said. He added that the past warming of the Arctic couldn't possibly be blamed on greenhouse gas emissions since it occurred long before the industrial era.
It is true that Arctic summers
were warmer during the period 4,000 - 7,000 years ago. The mean July temperature along the northern coastline of Russia may have been 2.5 to 7.0 �C warmer than present, and Scandinavian summer temperatures were 1.5 to 2 �C higher than at present. The warming was caused by changes in the amount of sunlight the north pole gets in summer due to variations in the Earth's orbit.Figure 1
. Summertime temperatures in the Arctic during the Mid-Holocene Warm Period (about 6,000 years ago), compared to today's temperatures. Image credit: NOAA
.Earth's orbital variations
Earth's orbit is not perfectly circular, which means that we are closest to the sun in December and farthest in July. Thus, the Southern Hemisphere gets more sunlight in their summer (December) than the Northern Hemisphere does in their summer (July). The Earth's current 23.5 degree tilt keeps the north pole pointed towards the star Polaris, which results in the north pole being pointed at the sun in December (Northern Hemisphere winter). The Earth slowly wobbles around its axis with a period of 23,000 years a href=(precession)
so that the pole star gradually changes with time. During the period 4,000 - 7,000 years ago, the pole star was Thuban in the constellation Draco, and the north pole was pointed at the sun during Northern Hemisphere summer. This resulted in much warmer summer temperatures in the Arctic, since there was more summer sunlight. Conversely, winter temperatures were colder, since the Earth was farther from the sun during the Northern Hemisphere winter. The Arctic will again be much warmer in summer 16,000 to 19,000 years from now when the cycle repeats and Thuban is once more the pole star. The reason the ACIA failed to mention this climate period is that the average temperature
in the Arctic remained about the same during the period 4,000 - 7,000 years ago; it was just the summers that warmed. The warming since 1980 and the 1930s were warmings over all seasons, so it was misleading for the climate skeptics to compare Arctic temperatures 4,000 - 7,000 years ago with these modern warmings.
This is the second in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday. Next blog: Why the Arctic sea ice is shrinking.
Also, be sure to visit our new Climate Change
blog, written by Dr. Ricky Rood of the University of Michigan.