Thursday, September 29, 2016

CO2 and other radiative forcings of global warming



In my last post I talked about the gases that are known as "greenhouse gases," that is, the ones that control the temperature of our planet. The three main greenhouse gases - water vapor, CO2, methane do virtually all the work even though they make up only a very tiny part the atmosphere. And as I said already, water vapor (H2O) is actually the biggest contributor to the greenhouse effect (between 66% and 85%), but its effects are generally short-term because if too much gets in the atmosphere it just rains or snows out. [It's a bit more complicated than that so I'll go deeper into water vapor in a future post, but this is sufficient for now]

I had intended to go right into the importance of CO2 and methane in this post but it became clear that a quick discussion of radiative forcings was first necessary (I'll go into feedbacks in a future post). Please click on the links for additional description of each term mentioned. In short, radiative forcings are whatever factors that drive or "force" the climate system to change. Some are natural, some are not. And some are a bit of both.

I've already mentioned the main greenhouse gases that are forcings: CO2, methane, and to a lesser extent halocarbons (e.g., refrigerants), nitrous oxide (laughing gas), ozone, aerosols (small droplets), and contrails from airplanes (the white "clouds" you see behind the plane). Another forcing is the land surface albedo, which is the amount of light reflected off the land surface. For example, the whiteness of ice and snow has a high albedo because white reflects better than, say, the dark coloring of lakes and oceans. Humans impact the land surface albedo by deforestation (which increases the albedo) and industrial activity that deposits black carbon on snow (which decreases the albedo). Overall, the total land surface albedo-based forcing is a net cooling and minor compared to the effects of greenhouse gases. You can read more about the basics of albedo here since it's an important concept we will come back to later. 

The chart below shows the relative contributions of the main forcings (IPCC, AR5, WGI, Fig. 8.20). Bars extending to the right mean warming, bars to the left signify cooling. It's easy to see that CO2 is the biggest driver of warming. The yellowish bar near the bottom is the total anthropogenic, i.e, man-made, contribution to warming.



There are a couple of other forcings that we don't have any control over - solar intensity (or solar irradiance) and volcanoes. The brightness of the sun varies naturally over time.  Sunspot cycles of about 11 years are the most obvious variation in solar intensity. There is also the ultraviolet (UV) light intensity, which would be absorbed by ozone and contribute to warming. But the planet's temperatures have been increasing with no correlation at all to the 11-year sunspot cycle, and if it were UV it would mean the upper atmosphere would warm first, and that is the opposite of what is actually happening. In fact, solar intensity has been decreasing for decades while temperatures have been rising. You can see from the bottom bar in the graph above that solar irradiance actually has been contributing a slight cooling effect, far overwhelmed by the warming effects of CO2 and other greenhouse gases. Take all together, the totality of the data clearly demonstrates that it is not the sun that is causing the warming we observe.

Finally, as we all know, volcanoes don't erupt very often, especially big ones that can impact climate for a significant period of time. When they do happen they inject both CO2 and sulfate aerosols into the atmosphere. The warming effect of the CO2 emitted from volcanoes competes against the cooling effects of aerosols and particulates, so most volcanic eruptions actually have a net cooling effect. That effect only lasts for days or weeks. Even the incredibly strong eruption of Mount Pinatubo in the Philippines in 1991 only impacted climate for a couple of years. Also, the amount of CO2 emitted by volcanoes is only about 1% of what is emitted by human activity, so volcanoes aren't causing our climate change, humans are.

Okay, enough with the forcings already (yeah, I can hear you say it). In the next post we'll take a look at CO2 and why it is the primary driver of man-made climate change.




 [This is part of a series of posts explaining the basic science of climate change. More posts will be added weekly.]


 
 

Thursday, September 22, 2016

So What are the Greenhouse Gases...and What are NOT Greenhouse Gases



In my last post I talked about the greenhouse effect, which is a normal part of our atmosphere and the phenomenon that keeps the planet at a temperature that allows life as we know it. This is scientifically established, like gravity. Keep in mind that we are not talking about anything humans have done at this point, just what nature itself does. The greenhouse effect is basic physics. And it is natural.

Today I'll take a closer look at the gases that make up the atmosphere and which ones actually create the greenhouse effect. This seemed to be a sticking point for some folks who, whether intentionally or unintentionally, exhibited confusion over the basic atmospheric processes that make our planet livable.  

Take a look at the pie charts below.




As you can see in the top pie (purple, blue, and yellow circle), the dry atmosphere is about 78% nitrogen (N2), 21% oxygen (O2), and about 0.9% argon (Ar). In total, these three simple elements make up about 99.96% of all of the gases in the atmosphere. And their combined impact on the greenhouse effect is - drum roll please - zero, zilch, nada, nothing. Nitrogen and oxygen are not greenhouse gases and thus do not contribute to the greenhouse effect. 

Now look at the bottom pie above (the mostly light blue one).

Only about 0.04% of the atmosphere controls the temperature balance of the earth. And most of that 0.04% is carbon dioxide (CO2), which is indeed a greenhouse gas. Other greenhouse gases in trace amounts including methane (CH4), nitrous oxide (N2O), and ozone (O3). Finally, you can add in water vapor (H2O), which is not included in the dry atmosphere totals above because it varies, but it on average is roughly 1% of the atmosphere.

So to reiterate, this approximately 0.04% of the atmosphere controls the entire natural greenhouse effect that keeps this planet at temperatures conducive to animal, plant, and human life as we know it.

But all greenhouse gases are not created equal. They vary in how efficient they are in absorbing long-wave radiation coming back up from the Earth. When you rank them based on their relative contribution to the greenhouse effect you get:
  1. water vapor
  2. carbon dioxide
  3. methane
  4. ozone
Oops. Isn't carbon dioxide (CO2) supposed to be THE most important greenhouse gas? Well, in a way it is, but it actually isn't the biggest contributor to the greenhouse effect. Water vapor actually generates more greenhouse effect than any other single gas. Estimates vary, but the combined effects of water, both as a gas (water vapor) and as a liquid (droplets in clouds), contributes between 66% and 85% of the greenhouse effect.

So why don't we talk more about water vapor? Well, besides some really technical stuff that I won't go into here, water vapor is naturally cycled into and out of the atmosphere on a relatively short time cycle (think, rain and snow). This means that while it has powerful short term impacts on temperature (a cloud passing in front of the sun will immediately make it feel cooler), water vapor is not a major driver of long-term climate change. It's more of a feedback, whereas CO2 is a forcing. (I'll talk about what that means in the next post) To find out more about the role of water vapor, check out this entertaining but informative video (notice the repeated cameo by Carl Sagan). For different reasons, ozone also has a pretty limited effect (and in fact actually contributes a small net cooling). Therefore, the concentrations of CO2 and methane are the main drivers of greenhouse gas induced climate change.

Remember, we have not said anything yet about human activity. Everything about the normal greenhouse effect is basic physics and natural. It is what keeps our planet "just right" (as opposed to too hot like Venus or too cold like Mars).

In the next post I'll go into more detail on CO2 and methane and their contributions to the greenhouse effect. I'll also look at why they are so important to future climate changes.



 [This is part of a series of posts explaining the basic science of climate change. More posts will be added weekly.]