Rashid's Blog: A Place for Inquisitive Souls

Climate forcing is a term you hear bounced around a lot among academics and politicians. Basically what it means in relation to climate science is the amount of energy that something contributes or takes away from our current energy exchange in the greenhouse effect. It is usually measured in a certain amount of energy (Watts) over a certain amount of area (meters squared). This is really important in determining what effects each thing has on our current global temperatures and how much each thing contributes to climate change. Radiative forcing takes into acct. the change in the amount of a substance and its energy influence. For example, even though we as a society may be spitting out tons of CO2, if CO2 has a small forcing it may not make much of a difference to overall climate. However, if CO2 has a large forcing, then it would make a much bigger difference.

A climate forcing can be positive or negative. Things that contribute to warming (i.e. increasing amount of energy/heat in our atmosphere) have a positive forcing value. Things that contribute to cooling (like pollution that causes clouds to form and reflect sunlight away) have a negative forcing. If we add up all the forcings of all the things that affect climate, then we get the total net forcing on the entire Earth system. The trick here though is to figure out all the things that would have a climate forcing, and then to figure out what that number is and what the uncertainty (possible error) of that number is. If our total net forcing is positive, then we can numerically show how much increased energy is being added into our earth system, and use this number to help develop models to make predictions for the future. If our total net forcing is 0, then it would stand to assume that the things that contribute or take away from global warming have balanced each other out. If our net forcing is less than 0, then it would mean the earth is losing more energy than it is taking in and one would expect a cooling to be occurring. So really, no one knows about the effect and magnitude of anything that could contribute to climate change until they take into acct. how much forcing each thing has.

So what are the climate forcings of various climate affectors and how certain are we to their values? This chart comes from the IPCC 4th Report Summary for Policy Makers (see pg. 4 for better resolution, this pic taken from realclimate.org, but it’s a reprint of the IPCC chart), and shows radiative forcing values of these components since 1750 (start of industrial revolution):

So you can notice a number of things about this chart, there is actually quite a lot of information on it. The RF terms means “Radiative Forcing” terms. Anthropogenic means made by people – things human activity has contributed to the atmosphere. The RF values are the actual forcing values for each term. Spatial scale I think is pretty explanatory, and LOSU stands for “level of scientific understanding” – this term was decided on by the authors of the chapter based on the scientific literature available, and is their assessment of how well we understand the processes and are able to calculate the values of each of these terms.

A little bit on each of the terms:

GHGs: One of the most noticeable things on this chart is that CO2 and other greenhouse gases like methane (CH4), nitros oxide (N2O) and hydrocarbons have a very large contribution to net radiative forcing. Different gases are prevalent in different amounts (you can view their concentrations in the air here (IPCC 4th report, Ch.2)- and have differing radiative capacities, often expressed as global warming potential (GWP) in terms of carbon dioxide equivalents. Carbon dioxide equivalents are how much carbon dioxide it would take to have the equivalent heat absorbing capacity as the other greenhouse gas. I’ll try to make a list here to see but my list making skills in wordpress are somewhat limited:

CO2 – current concentration 380 ppm (parts per million), GWP =1
CH4 – current concentration 1775 ppb (parts per billion), GWP = 20
N2O – current concentration 320 ppb, GWP = 310
CFCs – current concentrations very small, but GWPs can be as high as 8,000

So the radiative forcing takes into acct. the change in concentrations as well as their radiative absorbance capacity in order to compute the amount of climate forcing each has had on our atmosphere over the last 250 years.

Aerosols: Also noticeable are the very large error bars on aerosols. Aerosols have shorter lifetimes so are harder to get an average measure of their concentration. Unlike the GHGs, aerosols are not necessarily evenly spread out through the atmosphere, which complicates our ability to understand their exact contribution. Also, aerosols can participate in a number of mechanisms Aerosols can reflect sunlight back to space. They can also can help to seed cloud formation. Clouds have a high albedo (i.e. they are white), and so reflect light back to space as well. There are also a number of various chemistry reactions that aerosols can take place in the atmosphere that can have a number of effects, not just on climate. Aerosols also are not just one specific molecule like CO2, but can be a number of different chemical compounds from a number of different sources. This pic from NASA shows where aerosols come from pretty well:

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