Monday, April 9, 2007

Sustainable Planet

A report issued Friday from the UN-sponsored Intergovernmental Panel on Climate Change (IPCC) has once again prompted debate about how to respond to global warming. Perhaps one of the biggest obstacles to meaningful progress on this issue comes from the perception that changing our consumption levels will drastically reduce our standard of living. This kind of hysteria is fueled by the commonly quoted phrase: "if everyone consumed like North Americans, we'd need five planets to support us". But if you consider the natural resources, population, and consumption patterns of the U.S. by itself, how unsustainable are we, really?

This is similar to the question raised by popular "ecological footprint" calculators, but it takes into account the fact that our nation has more natural resources and a lower population density than most of the rest of the world. So in a sense, it's more fair. Basically, "sustainability" is a careful balance of three factors: (1) the available natural resources, often lumped into a quantity called "biocapacity", (2) the average consumption level of those natural resources, known as our "footprint", and (3) the total size of the population. Since total biocapacity is fairly constant, any increase in population erodes the available footprint per person. So if we want to maintain a certain standard of living, we need to either control population growth or decrease our footprint over time.

As you can imagine, the average footprint in the U.S. is actually growing slowly over time, mostly from energy use (the inability of the environment to absorb all of the carbon dioxide we release). The last time we were in balance with the available resources was around 1969. As it stands today, the average footprint is about twice the available biocapacity per person. So we only need two planets to be sustainable as a nation, not five. The reason is simple: it's harder for Africa to be locally sustainable because they don't have enough arable land, and it's harder for China because they have too many people. We should certainly help them address these problems, but those challenges are distinct from the challenge of sustainability at home.

Even so, the average footprint in the U.S. is still twice as high as it needs to be if we want to live a sustainable lifestyle. So you might ask: when in history was the standard of living in the U.S. half of what it is today? Even if you use a traditional economic measure of the standard of living, like "real GDP per capita" (which is adjusted for inflation and population growth), you find that to be sustainable we would need to adopt the lifestyle of the mid-1960's. That doesn't seem like such a sacrifice, and it's close to the "1969" balance mentioned above because improvements in (agricultural and energy) efficiency have basically made up for all of the population growth since then.

The average U.S. footprint is currently about 24 acres, while the locally sustainable level is around 12. Something to shoot for.

Monday, April 2, 2007

Blaming the Sun for Global Warming

In recent weeks, skeptics of the notion that global warming is caused by humans have been promoting alternative theories. One of the most plausible explanations asserts that global warming is part of a natural cycle caused by changes in the temperature of the Sun. It's undeniable that the Earth experiences natural variations in climate due to the Sun and other factors, but the warming over the past 100 years has a fundamentally different character, and is unprecedented in the 600,000 year climate history that scientists have reconstructed from ice cores and tree rings.

One way to quantify the relative importance of natural variations compared to those caused by people is to calculate the climate "forcing" of each factor individually. In this way, scientists have shown that changes in the concentration of carbon dioxide (CO2) in the Earth's atmosphere are at least 5 times more important to global warming than any changes in the Sun. The skeptics argue that the atmospheric CO2 concentration is changing because of temperature changes on the Earth, rather than the other way around. In fact, scientists have shown that changes in the Earth's orbit over thousands of years lead to natural changes in global temperature (periodic ice ages) that also cause natural changes in the CO2 concentration. But this predictable effect cannot explain the extraordinary warming that the Earth has experienced in recent times.

Another point raised by skeptics is that for several decades after 1945, global temperatures were falling even though the atmospheric CO2 concentration was rising. This is true, but there is a very simple explanation: in addition to the warming effect of CO2, common pollution in the form of particulate matter called "aerosols" has a cooling effect on the climate. Prior to the clean-air legislation enacted in the 1960's and 1970's, the cooling from aerosols overwhelmed the warming from additional CO2. As developing countries such as China and India adopt similar clean-air measures, this may actually accelerate global warming in the future.

By raising plausible doubts about the responsibility of humans for global warming in modern times, skeptics are trying to confuse the public into inaction. If climate scientists successfully communicate these subtle effects to the public, the skeptics will ultimately fail.