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Why Is Earth Warming?

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


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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.

Kevin Trenberth About the Author: Dr. Kevin E. Trenberth is a Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research. He has been prominent in all aspects of climate variability and climate change research and is a leader in the Intergovernmental Panel on Climate Change assessments and in the World Climate Research Programme. In recent times his primary research has focused on the global energy and water cycles and how they are changing.

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






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  1. 1. da bahstid 12:31 pm 08/19/2011

    Even though an unwittingly small but extremely boisterous minority loves to jump in crying foul about conspiracy theories involving Al Gore, secret societies, cosmic rays and Eywa, it’s never a bad thing to frame the overall picture of the actual science in a more comprehensive scope for the lay person when it comes to complex topics like this.

    It’s only natural that people are used to understanding temperature only within the range we experience on a daily basis (meaning 0-100F). Thinking back to how much time it took our teacher to cover the concept of absolute zero in high school physics (and this was an honors class) and how little the average American retains from the high school in the first place, i really don’t think the average person has any idea that the sun warms Earth’s surface by some 500 degrees, while greenhouse magnifies that by only about 10% or so (very rough numbers) In the big picture, a 40% increase in CO2 even for its relatively low percent concentration causing a few degrees of warming should be so easy to understand. It’s a paltry increase in astronomical terms, but in terms of ecosystem stability it’s enormous.

    You can’t do anything about the people that insist on getting their science “education” from political lobbyists, the fact that they choose to do that in the first place sais all that needs to be said. But for most people sitting on the fence who just didn’t happen to care that much for high school physics at the time, putting everything in the correct perspective is so incredibly important.

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  2. 2. deepinelastic 6:49 pm 08/19/2011

    I got my education to the PhD level in Physics at the University of Texas and Columbia University in New York and I am appalled at your misleading reading of “Why is Earth Warming?”. Your perspective is the one that is out of whack, not the high school physicist. Your bald-faced statement that human generated Carbon dioxide (~3% of the total) is causing an energy imbalance in the system and you know because you have a climate model. It would be nice if your climate models agreed with each other and reality better that a factor of 2 – 10. Your not humble enough to realize that you don’t understand many of the factors you quote with such certainty, water vapor, clouds, cosmic ray generation of water droplets, or the mechanics of El Ninos and La Ninas which have much better correlations with the actual temperature than CO2 which is, embarrassingly for you, almost uncorrelated with the temperature changes. You make conclusions when you know the data is insufficient. It seems as if you make high school calculations to explain bad weather, hurricanes, tornados, rain, droughts, as due to global warming when you can’t even put thunderstorms into your models. The only reason your models are even within a range of 2 – 10 is because they are calibrated to behavior in the recent past so that they haven’t had time to deviate any more. Keep working at it, it’s important, but be humble enough to know what you don’t know. Your going to lose your funding if you keep putting out what you think the media wants to hear.

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  3. 3. Postman1 9:22 pm 08/19/2011

    deepinelastic – Thank for your informative and intelligent comments. You and a few others of like mind, are the only reason I ever bother to read this site these days.

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  4. 4. MikeBike 10:04 am 08/20/2011

    “You make conclusions when you know the data IS insufficient,” claims Deepinelastic. I guess PhDs aren’t what they used to be. Are written dissertations even required? Apparently not. Makes you wonder about the writer’s credibility.

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  5. 5. da bahstid 11:28 am 08/20/2011

    I would suggest clicking on the link the author provides. And while at it, read through the entire site. One of the things it bothers to do is refute, one by one, skeptical claims of inaccuracy, insufficiency, conspiracy, etc.

    The people running that site obviously have tremendous amounts of data available to them, drawing on the cumulative publications of many PhDs in the field. It shows very clearly the amount of effort and accountability that is exercised in real scientific research. It just makes it that much more comical when you then read the assertions of the skeptics, which are practically juvenile in their formulations.

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