Guest post by Erl Happ
This post was generated in response to the Christopher Monkton thread. It is not a criticism of Christopher Monkton but of our tendency to imagine that artful mathematicians (I am not one) are sufficiently sophisticated to deal with complex problems. Indeed the debate as to the value of feedback processes illustrates the lack of utility of mathematics when unconstrained by observation of the real world. Climate Science is full of it.
As I understand it the proposition goes like this:
Enhanced GG composition, more back radiation, enhanced evaporation, more cloud and IF cloud enhances back radiation, the surface warms. The enhancement of cloud density depending upon the IF supposedly represents the feedback.
But cloud reflects incoming energy. The feedback notion requires that the loss of energy to the surface due to cloud reflection of incoming short wave radiation is outweighed by the increase in energy trapped in the ‘below cloud level system’ due to cloud returning OLR to the surface. That’s the IF factor again.
The IF proviso requires that evaporation from the surface not only keeps pace with the increase in surface temperature. It must exceed it for cloud density to be enhanced as the surface warms.
There is a little logical problem here. If the feedback from long wave radiation exceeded the value of the reflected short wave, the oceans would soon boil. That problem is sidestepped by suggesting that it is only the high ice cloud that is important in the feedback. So, in the end the result depends upon the mix in the categories of clouds that provide net reflection versus those that provide net surface warming and whether the moisture supply to the atmosphere keeps up and somehow tips the balance towards those clouds that are supposed to provide a net warming effect .
This is already too complex and includes unknowns that are unquantifiable.
Now, lets look at the real world. Consider:
Do clouds warm the surface? Logically, if clouds had that effect, with more clouds the surface should warm. But near surface clouds arrive in warm tropical air. It’s warm because of its origin. The warmer and wetter it is the more the precipitation. This warm moist tropical air produces cloud and precipitation strictly in proportion to the chilling it receives. Warm that same air and the cloud disappears. (The Foehn effect). Precipitation enhances the supply of moisture at the surface cooling the surface. The air is in constant movement and the system is mind bogglingly dynamic. But one constant is the decline of surface temperature as we move from equator to pole. Satellites show that warm moist tropical air travels all the way but is dried as it moves. Hence the polar latitudes are cold deserts with the air in these regions containing little moisture that remains to be precipitated producing a gradually accumulating mass of ice in perennially sub freezing temperatures. Lesson: The presence of low clouds reflect very recent change in air temperature and is unrelated to the supply of moisture to the atmosphere from the surface. The presence of these clouds depends upon the supply of energy to the tropical ocean and the direction of the wind.
In mid latitudes the atmosphere between 600hpa and 100hpa (where the ice cloud called cirrus and stratus is located) responds in terms of its cloud cover to a moisture supply from places remote to the point of observation. (tropical convection, polar frontal action). Supply is relatively invariable and as a result cloud comes and goes according to flux in the temperature of the upper troposphere. Temperature in this zone is a function of ozone content and depends upon stratospheric processes. In the mid latitudes the troposphere above 300hPa contains appreciable ozone and peaks in temperature in mid winter when outgoing radiation peaks. At this time the surface reaches its seasonal minimum temperature. Radiation peaks in winter due to the enhancement of the high pressure cells of descending warming air in the winter hemisphere. The temperature of the cloud bearing layer does not relate at all to change in surface temperature. If radiation increases the presence of ozone ensures that the air warms and the cloud disappears.
For cloud to increase as the atmosphere warms it requires that evaporation is enhanced as the surface warms so as to enhance relative humidity promoting enhanced cloud cover. This proposition is tested once a year in the northern hemisphere. Because of the preponderance of land which is opaque to short wave radiation (unlike the sea) near surface air temperature increases strongly. In effect the surface returns warmth to the atmosphere by conduction and radiation. The convective process of heat loss via decompression (that we see in the tropics) is inoperable because of an insufficiency of moisture supply to the atmosphere. Transfer by conduction and radiation is therefore enhanced and the entire troposphere warms.
We see here that vvaporation fails to promote the addition of sufficient moisture to the atmosphere to maintain cloud cover. So, cloud falls away and global air temperature peaks in July in conformity with this strong seasonal influence driven by the accident of geography which is the northern hemisphere. A potential runaway feedback system that is the exact opposite of that posited above (warming surface more cloud) is curtailed by the passage of the Earth around the sun while it spins on its tilted axis.
In January, when the suns irradiance is 7% stronger due to orbital considerations global near surface air temperature reaches its minimum because global cloud cover peaks. Taken in its entirety, cool the Earth’s atmosphere and cloud increases. The surface cools. It will cool in the face of enhanced radiation.
Summarizing: Does the presence of cloud result in surface warming? No. In January, global cloud cover is 3% greater than July. Irradiance 7% greater. Surface temperature 4° cooler. Will a warmer sun heat the Earth? Not necessarily. It depends upon what happens to the cloud. If there were less land and more sea the ocean would gradually warm.
The proposition that cloud is enhanced as the near surface atmosphere warms is also testable by looking at historical data for precipitable water as the globe has warmed. Reanalysis tells us that it actually falls away.
The Earth system also demonstrates what happens when additional greenhouse gas is added to the troposphere. This happens in the coupled circulation over Antarctica. The system waxes and wanes according to the activity of the night jet in modulating the ozone content and temperature of the upper stratosphere. The convection that results involves warmer ozone rich air (10ppm) ascending. Relatively ozone poor stratospheric air (say 7ppm) descends into the troposphere (naturally containing ozone at the ppb level) that in consequence becomes ozone rich. The consequence is gross warming of the troposphere on the margins of Antarctica and the generation of the lowest surface atmospheric pressures on the planet. The flux in pressure in this zone depends simply upon the rate of ozone churn into the troposphere. Ozone is carried towards the equator by the counter westerlies destroying cloud as it moves by virtue of its greenhouse gas property. It absorbs at 9.6 micrometers.
As this greenhouse gas is added to the troposphere cloud cover falls away. The surface temperature feedback is due to enhanced shortwave radiation, not longwave retention. This too is a potentially disastrous feedback scenario that is limited by the fact that the ozone content of the stratosphere varies within limits and the Earth’s surface is mainly water which soaks up energy without adding a lot of moisture to the atmsophere. Given enough time, the feed rate of ozone peaks and shortly after atmospheric moisture and cloud cover recovers.
The prime source of long wave radiation emanating from the Earth system is the high pressure cells of the winter hemisphere where the air warms by compression as it descends, a cloud free zone promoting surface warming when it is most needed…………..despite the abundant long wave radiation streaming out to space.
Conclusion : Cloud cools.