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Why Is Earth Warming?

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Questions come up all the time about global warming, climate change and the role of human activities. For instance we are asked: "Is global warming from an energy imbalance in which the planet is warming up, or is it due to a redistribution of energy within the system?" The following is a brief answer.

Yes, the Earth is out of balance. This write up of mine provides details.

Essentially the imbalance comes from the changing composition of the atmosphere from human activities: more carbon dioxide and other greenhouse gases and thus more greenhouse effect. Carbon dioxide concentrations in the atmosphere have increased more than 40% since pre-industrial times (say 1750) and more than half of that is since 1970!

The imbalance is estimated as 0.9±0.5 W m-2. This is compared to the normal flow through the climate system of 240 W m-2. Or adding these up for the total area of the Earth, the natural flow through the system is 122 PetaWatts (122×1015 W) and the imbalance is estimated at 0.45 PW. These numbers can be compared with the biggest power stations which are of order 1000 MegaWatts, and so the energy from the sun is 122 million of these power stations. Add up all power stations and energy usage from other sources including non electrical, and it is still a factor of 9,000 larger. This calculation highlights the fact that the main way humans interfere with the climate system is not by competing with the sun directly, but rather by interfering with the natural flow of energy through the system. Some local competition occurs in big cities and gives rise to the urban heat island effect.

However, the imbalance is the result of both the increased trapping of energy and the response to that: the planet has warmed and is trying to radiate more. But it is in catch up mode and that takes a long time because the ocean response is slow. The net effect of human change is about a 1% effect and the 0.9 W m-2 is the end result.

Certainly the redistribution of energy can also play a role. It does so with El Niño events and La Niña events. The ocean warms up during certain stages of La Niña, and the ocean gives up heat and causes a mini-global warming in latter stages of El Niño events. But for overall warming to come from the ocean, other parts of the ocean would have to be cooling off. That happens locally and temporarily but overall the oceans are clearly warming, as shown by direct measurements of temperature. That means the ocean expands, and one consequence is sea level rise.

Another consequence of warming is melting of land ice (glaciers, Greenland and parts of Antarctica) and that too adds to sea level rise. Melting Arctic sea ice is also a manifestation but does not add to sea level rise. Since 1992 when precision altimeters were deployed in space for the first time, global sea level has risen at over 3 mm/year: or 2 and ¼ inches (a rate of over a foot per century). So the ocean is warming all right, there is no doubt, and the warming corresponds closely to the estimated imbalance because that is where most of the energy ends up.

How do we know what the imbalance is? The main way is to add up all the observed changes noted in the previous paragraph. Over 90% of the warming of the planet is found in the oceans. Some goes into melting glaciers, sea ice, Greenland and Antarctica. A tiny amount goes into warming the land and the atmosphere. We can also make estimates using global climate models and compare with observations. The advantage of the latter is that the model “observations” are perfect and energy is conserved, while some energy can be lost or go un-observed in the real world; the latter is the case in the deep ocean for instance, which appears to be heating up although data are few.

In recent times, we can also make measurements from satellites of energy coming in and going out of the earth system. These measurements provide good estimates of changes from year to year but are not accurate enough to pin down the actual imbalance. Together these observations indicate how well we can close the energy budget so that everything adds up, and this proves difficult in the short term of a few years, but a coherent picture is present over the longer term, say since 1992 when we have global sea level as an extra constraint.

The views expressed are those of the author and are not necessarily those of Scientific American.

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