Solar Radiation, Co2, and Global Climate

Shauna Gamble

12/3/13

Geology 103 – Steven Newton

Solar Radiation, CO2, and Global Climate

In the last ten years, scientists have measured increased solar radiation levels bombarding the globe.  While solar radiation is generally a constant, the quantity of solar radiation that reaches the Earth’s surface is affected by a number of different factors.  One possible reason for the increased measurements of solar radiation could be the reduction in total air pollution throughout the developed world.  The global warming ability of solar radiation might partially offset the gains in climate stabilization from reductions in CO2, increasing the amount of CO2 that must be removed from the atmosphere to create a measurable impact in global temperatures.  

                Solar radiation or “sunlight” is a fundamental component in the energy cycle of the earth.  Solar radiation is responsible for much of the heating of the earth, so its role in climate change and regulation is indisputable.  Not all solar radiation that reaches the earth is able to get to the surface, as much of it is deflected or absorbed by gases in the atmosphere.  Air pollution, natural pollutants like volcanic gases and particulates play a role in this deflection.  Greenhouse gases like CO2 simultaneously deflect solar radiation as it enters the atmosphere and as it leaves, trapping heat in the earth’s atmosphere through a phenomena called the Greenhouse Effect.  Because of the duality of greenhouse gases in simultaneously warming and shielding the earth, it is difficult to measure the impact of removing these gases from the atmosphere.

Solar radiation is not a constant; it actually varies both due to the vagaries of the sun’s actions and because of human effects.  Global dimming and global brightening have been examined in the past decade, as interest in climate regulation has increased throughout the world.  Unfortunately for long-term predictions of solar levels, there were no concerted attempts to measure global solar radiation consistently until 1957 with the establishment of the Geophysical Year. (ETH Zuric, 2009)

        Solar radiation is actually relatively variable, with the level of radiation measured as solar irradiance.  While the total variation is not severe, it is measurable and patterned over the last 30 years.  According to most astronomers and scientists, the sun follows an 11-year solar cycle with peaks in solar irradiance measuring around 1367 W/m2 every 11 years, and valleys intermittently at around 1365.5 W/m2. (Haigh, 2010)  These are small variations in solar radiation, and indeed are only detectable with modern detection equipment.   This is the amount of radiation that reaches the outer atmosphere of the earth, and it is estimated that the amount that makes it to the earth’s surface is around 0.1% of this total. (Lean, 2000)  The measurement of solar radiation’s absorption into the earth’s surface is even more shrouded with estimate and guess.  “As of now, researchers know that the atmosphere absorbs between 20 and 25 percent of the TSI and that the land absorbs 45 to 50 percent. With solar radiation, a 5 percent difference is huge…. 1 percent would throw off climate models of global warming.” (NASA Earth Observatory)

        The short term of global solar level measurement has led to some incorrect hypothesis about the trends behind solar radiation changes.  In the 1990s, scientists noted a 2% decrease per decade of solar radiation levels on the earth’s surface, and they attributed this to a global dimming of the sun’s solar radiation.  When data became available for a larger time period, however, those same scientists retracted their earlier hypothesis in favor of a theory that the sun’s radiation levels fluctuated based on a cyclical basis. (ETH Zuric, 2009)

        Some of the best research on solar radiation and the earth has come from NASA, funded mostly in connection with research projects to determine long-term climate changes on the planet.  NASA’s Earth Observatory has been monitoring solar variability for 22 years, and some interesting data has come out of the NASA sponsored Intergovernmental Panel on Climate Change.  For one, it is thought that the imbalance of solar radiation and outgoing thermal radiation is the primary cause of global warming, and will result in the earth increasing in temperature by several degrees over the next ten years. (NASA Earth Observatory)  NASA does cushion its statement with a few warnings about the complexity of interactions between the earth’s atmosphere and solar radiation.  For example, solar warming could decrease low level stratocumulus clouds while increasing cirrus clouds, creating a cloud cover pattern that could further increase global warming by trapping more thermal energy while reflecting more sunlight.  Pollution would also trap heat at the earth’s surface, but it could also have a positive effect, “an increase of sulfate aerosols created by pollution would likely reflect more sunlight and perhaps also make clouds more reflective, thereby countering global warming especially near pollution sources.” (NASA Earth Observatory)

        Since the research into global dimming and brightening is still in a beginning stage, there is little in the way of consensus as to whether or not it exists, or whether it plays a role in climate science.  According to the American Geophysical Union, global dimming remains a concern of climate scientists on more of a theoretical level than a practical level, but its impact on climactic measurements cannot be underestimated. (Haywood, 2011) While modern instruments are sensitive, the amounts of solar variance that they must measure are so low that the instruments have room for error.  This creates uncertainty towards solar measurements, and requires some statistical analysis of the results to create noticeable patterns.  According to NASA, the measurements of solar radiation have to be extremely precise because relatively small amounts of variance in either direction could cause huge changes in the climate, making the low confidence level of the data somewhat problematic. (NASA Earth Observatory)