Sunday, January 4, 2009

Historic Solar Irradiance

The intensity of the sun varies over time. Two differant researches have developed a record from the year 843 to 1961; Yang and Bard. As can be seen, they agree fairly well, but diverge slightly during the oldest time period.

Since 1978, there are precision instruments on satellites that can measure the changes that occur over time. The 11 year sunspot cycle is evident by these measurements. A long term project of mine is to build a simplified climate model that includes solar variation, CO2, CH4, areosals levels and El Nino/Southern Oscillation to crudely model averge global tempertures.

Saturday, January 3, 2009

Sugarcane and Climate Change

25 million years ago, a new type of plant evolved on earth. It started using a photosynthesis process called C4, which is distinct and more efficient than other plants. Examples of this type of plant include sugarcane (pictured above), maize, sorghum and switchgrass. These plants grow very quickly and are more efficient at using CO2 while being resistant to droughts and high temperatures. They are concentrated in the tropics (within latitudes of 45°). Many herbivores can not easily digest this new type of plant and it gradually spread throughout the earth. By about 5 million years ago, C4 plants became ecologically significant and started reducing CO2 levels.

Other things were happening at the same time. Ithmus of Panama formed and the Himilayian Mountians grew higher. Hard to say which was dominate, but the Earth gradually cooled and this ushered in a new epoch on earth with periodic ice ages. The ice ages have not been permanent as periodic variations of earths orbit around the sun have allowed the ice to melt and warm periods to return for brief (10K year) periods. At first, the ice ages occurred every 41K years. But as CO2 levels continued to fall, the ice ages lasted longer and the warm periods occurred less often.

Friday, January 2, 2009

Happy Perihelion

Not quite yet I know, but as everyone else is wishing happy New Years, it seems to be the season to wish good will.

On or around the 4th of January we (earth that is) will be closest to the sun. That the calendar year just started is only a coincidence. Due to orbital variations, our date of closest approach varies a little bit over time. That is the time/season of perihelion will gradually shift over thousands of years. Earth's axis is slowly but continuously changing, with a cycle of approximately 25,765 years.

Currently the distance between the earth and sun varies between 98.3– 101.7% of its average distance. At its average distance sunlight amounts to about 1365.5 watts/m^2. Being at perihelion, the sun’s intensity is greater of course. However, since intensity varies by the inverse square of the distance, its intensity is now about 1412.3 watts/m^2. That is a 6.7% increase over where it was just last summer!

Ever wonder how our climate would be if perihelion occurred in June instead of January? It won’t happen for another 12,000 years or so, but when it does Northern hemisphere summers would be warmer and winters colder. Just the opposite will happen in the southern hemisphere and there is another difference too. It not just that most of us live in the north, but there is much more land than the south.

The extra land of the north provides a big platform for seasonal snow, and snow feeds back into the climate through the change in albedo. That is the amount of sunlight that is absorbed. Less snow means more sunlight is absorbed and more warmth. More snow leads to less absorption and cooler temperatures.

Thursday, January 1, 2009

Global Temperature Trends

This chart is from data supplied by the United States National Climate Data Center out of Asheville, North Carolina. It is composed from monthly data from land and ocean temperature measurements. The sharp spikes in the blue line correspond to El Nino (warm) and La Nino (cold) oscillations that periodically occur in the Pacific Ocean. El Nino/La Nina areas are just part of the Pacific Ocean, so it is rather surprising that they can shift global temperatures as much as they do. However, as can be seen on the chart, the most dramatic shifts last only a few months.

The red line is a rolling 12 month average to help smooth out the spikes. Notice, how even after averaging the data for 12 months there are still obvious oscillations that last about 4 years. This illustrates that besides the dramatic spikes, there are also longer term El Nino/La Nina trends that cycle about every 4 years.

To smooth out these longer term cycles, a 5 year running average is constructed in the green line. It smoothes out the El Nino/ La Nina oscillations fairly well. Coincidentally, the 5 year running average for 1979 was nearly equal to the average for the century.

In 1991, a large volcanic erruption occured at Mt Pinatubo. This cooled the earth for a few years and the cooling trend is almost noticeable, but complicated by the El Nino/ La Nina osciallaitons. Never the less, the long term warming trend over the last 30 years is clearly visible with only minor shifts.