Why do we have weather?

It all goes back to the fact that the heating and cooling of the atmosphere is not uniform. Some volumes of atmosphere get more energy than they lose, and must get rid of the excess. Others lose more energy than they get, and must somehow get more energy. Weather and ocean currents are Earth’s way of moving energy from places that receive more energy than they lose to those that lose more energy than they gain.

We are changing the heating and cooling of the atmosphere in a number of ways. Consequently, the details of how the atmosphere and oceans move energy around–and thus the weather–must also change.

The problem is not simple. Much of the energy gain and loss of the atmosphere is itself due to the same weather that moves energy around. But the chances that all the changes will balance out naturally are about like expecting a tornado to put things back the way they were before a hurricane hit.

Please note that when I talk about heating and cooling of the atmosphere I am not talking about the replacement of warm air by cold or vice versa. Rather, I am following a mass of air, often called a parcel of air, and looking at the changes in the energy within it. So what are the processes that change the energy content of the air?

If the air is in contact with a surface, be it ground, water, or snow, energy will flow from the surface to the air if the surface is warmer, or from the air to the surface if the surface is colder than the air. The surface is most likely to be warmer than the air if it is daytime, summer, and the surface is fairly dark. If it is winter, night, or the surface is very light in color (snow, desert sand) the surface may be cooler than the air. Further, land temperatures are strongly affected by the season and time of day. Water temperatures are affected mostly by season and by ocean currents. The presence of clouds moderates the cooling or heating of the land.

Energy, in the form of latent heat, also flows when water vapor evaporates or condenses. Thus the evaporation of water from the oceans adds energy to the air near the surface, while the formation of dew removes energy from the air. The condensation of clouds does not change the energy content of the air, but it changes latent heat into a rise (or a reduction of the drop) in temperature. This is a very important way of transferring energy from the surface to cloud height.

How can air gain or lose energy when it is away from a surface? This is where the so-called greenhouse gasses become important. Air is transparent to the part of the light spectrum we see, which is where the sun puts out most of its energy. This is important for us, as our eyes wouldn’t be much use otherwise. But there is a large part of the light spectrum we do not see. The most important part for weather and climate is what is called the thermal infrared. This is the part of the light spectrum, the colors invisible to our eyes, given off by objects at what we call room temperature. Although the major components of the atmosphere (oxygen and nitrogen) are transparent in the infrared, certain gasses emit and absorb very strongly in these colors. In particular water vapor, carbon dioxide, methane and ozone are able to absorb energy given off by the earth’s surface and emit energy to space. The emitted energy depends very strongly on the temperature of the gas. Thus the rate of energy loss at 100 degrees F is over 6 times that at 0 degrees F. Cloud droplets also emit and absorb energy in the thermal infrared.

Over all, then, the air gains energy near the surface, at low latitudes, in summer and during the day. It loses energy at high elevation, at night, near the poles and in winter. Although ocean currents succeed in moving a considerable amount of the energy gained from the sun near the equator to the polar regions, the vertical energy transport and about half of the horizontal transport is left to the atmosphere—hence weather.

We have been changing the way in which the earth absorbs energy from the sun at least since the origins of agriculture, since cleared fields normally absorb energy differently from forests. But over the last hundred years or so we have also dramatically changed the greenhouse gasses in the atmosphere. Carbon dioxide and methane have increased as our growing population has relied more on fossil fuel burning and also needed larger areas given to agriculture. Warmer air can hold more water vapor, so water in the atmosphere has increased as well. It is even possible that the particles we have put into the atmosphere have made clouds more stable, which could also increase the water vapor in the air and hence the greenhouse effect.

There is no question that we have changed the energy losses and gains of the atmosphere, or that these losses and gains are what drives the entire weather system. The disagreements among climate scientists are not in whether a change will occur, it is in the details of what that change will be. But expecting no change, when we are changing the processes that drive the whole climate system, is extremely unreasonable.