Quantifying The Greenhouse Effect In The Tropics

Dr. Hansen has told us that the very survival of “life as we know it” depends on reducing CO2 levels to 350 ppm (parts per million.)

http://maps.grida.no/go/graphic/greenhouse-effect

I calculated the greenhouse effect using the same radiative transfer model incorporated in NCAR’s weather and climate models. RRTM (rapid radiative transfer model) is the part of their climate model which calculates the downwards and upwards flow of shortwave and longwave radiation. In other words, it is the portion of the climate model which calculates the greenhouse effect. 

RRTM is freely available and easy to compile and use, other than the fact that it uses a proprietary compiler. In this study we will learn exactly what changes to atmospheric CO2 concentration do to the greenhouse effect in the tropics.

But first, a review of how the greenhouse effect works. The Sun emits shortwave radiation which warms the surface of the Earth. This causes the surface of the earth to emit longwave radiation upwards into the atmosphere. Most of this longwave radiation gets absorbed by greenhouse gases – primarily H2O and CO2. Those gases in turn emit some percentage of their absorbed energy back down towards the earth. Thus the atmosphere warms the earth. In places which have a lot of humidity, the atmosphere warms the earth considerably more than the sun does.

We all know that 90 degrees Fahrenheit feels much hotter in Houston than it does in Denver. There are several reasons for this.

• The water vapour in the air emits radiation towards your skin, making you feel hot.
• The humidity reduces the ability of your body to evaporate perspiration, and cool.
• The denser air transmits more heat conductively to your skin.

Consider the opposite case – the high desert at night. I have been camping in Canyonlands, Utah in July when it was too hot to sleep at bedtime, and too cold to sleep at sunrise. The temperature can easily drop 60-70 degrees F overnight from radiative heat loss. This happens because there is very little humidity (H2O) in the atmosphere to capture and re-emit longwave (LW) radiation.

By contrast, in cloudy/humid climates, it is not unusual for the daytime and nighttime climates to be nearly constant. San Diego normally has less than 15 degrees F variation between day and night.

In Antarctica, there is almost no water vapour, so temperatures can drop close to minus 130F – below the freezing point of CO2. There are very few greenhouse gases in the atmosphere over Antarctica.

Every place on earth has about the same amount of CO2, but there are huge differences in temperature. Thus it becomes apparent that it is H2O which dominates the greenhouse effect, not CO2. So let’s quantify that by running several RRTM experiments.

The greenhouse effect is measured by the amount of downwelling longwave radiation – measured in watts/metre². The use of the model is very straightforward. The user provides information about clouds, temperatures, and atmospheric composition for a given number of layers in the atmosphere. My calculations used no clouds, 60 layers of atmosphere, and a surface temperature of 299.38K. The atmospheric profile is the one provided by RRTM with this comment “TROPICAL ATMOSPHERE ADJUSTED TO RIDGEWAY’S GLA MODEL (CO2=355ppm).“

The first experiment is to calculate the baseline, i.e. the amount of downwelling longwave radiation at the ground surface in the tropics on a cloud free day – with 25,600 ppm H20, 394 ppm CO2, and an ambient temperature of 20C. Those are typical atmospheric conditions in the tropics. The amount of longwave radiation emitted from the surface is 397.08 watts per metre². This is our baseline.

Now let’s see what happens to downwards longwave radiation (LW) if we remove all CO2 from the atmosphere. Downwards radiation drops to 392.23 watts per metre², a reduction of 4.85. Thus CO2 is only responsible for 1.3% of the cloud free greenhouse effect in the tropics. This sounds low, but it is correct because almost all of the LW bands absorbed by CO2 are also absorbed by H2O. In the absence of CO2, the humid tropical atmosphere would absorb almost all of the LW normally absorbed by CO2. As you can see below, there is only one narrow gap in the H2O spectra at wavelength 0.00125 cm.

http://www.aer.com/scienceResearch/rc/m-proj/lbl_clrt_mls.html

Next let’s see what happens if we also remove all H20 from the atmosphere. Downwards radiation drops way down to 42.23 watts per metre². Therefore, H2O is responsible for 90% of the cloud free greenhouse effect in the tropics.

Next experiment is to reduce CO2 from 390 ppm to 350 ppm (Hansen’s magic number.)  Downwards radiation drops to 396.91 watts per metre² from 397.08 watts per metre². Hansen’s “earth saving” 350 ppm only reduces the cloud free greenhouse effect in the tropics by less than one tenth of one percent.

Next we “double” CO2 to the fabled 550 ppm. Downwards radiation increases to 397.68 watts per metre² from 397.08 watts per metre². The cloud free tropical greenhouse effect increases by less than 0.2% from a doubling of CO2.

Next we increase CO2 by 10X, up to levels similar to the Ordovician. Downwards radiation increases to 403.51 watts per metre² from 397.08 watts per metre².  In other words, increasing CO2 by 10X only increases the cloud free tropical greenhouse effect by 1.6%.

So how does this last number compare with changes in humidity? Let’s change the absolute humidity in the lower four atmospheric layers to 3%.  Downwards radiation increases to  407.57 watts per metre² from 397.08 watts per metre². In other words, a 20% increase in humidity has more impact on the greenhouse effect than a 1000% increase in CO2.

Conclusions : In the tropics, CO2 plays minimal role in influencing the radiative balance, and the first 20 ppm is responsible for almost all of that. Further increases in CO2 will have very little influence on the greenhouse effect in the tropics.

Roger Pielke Sr. has done similar studies and come up with similar results.

http://pielkeclimatesci.wordpress.com/2006/05/05/co2h2o/

1. The effect of even small increases in water vapor content of the atmosphere in the tropics has a much larger effect on the downwelling fluxes, than does a significant increase of the CO2 concentrations. Thus, the monitoring of multi-decadal water vapor trends in the tropics should be a high priority. While the increase in CO2 concentrations, and resulting increase in downwelling longwave flux can result in surface ocean warming, and thus increase evaporation into the atmosphere, it is the atmospheric water vapor signal that should be monitored for long term trends, as it is the dominant greenhouse gas that has the greater climate response.