The tipping point

The tipping point is the point at which something has gone too far — or past the point of no return. Think of slowly going up the first climb on a roller coaster. After you go over the top, no one can stop the ride.

Scientists believe that climate change has a tipping point, when the damage becomes too great to be reversed. After this point, not only can nothing reverse the impact on the planet, but little could stop that impact from increasing, either — it just keeps getting worse.

To determine the climate's tipping point, scientists first had to look at what would happen if, say, temperatures went up by, say, 3.6 or even 10.8 degrees Fahrenheit (2 or 6 degrees Celsius) above 1850 levels. (These temperature increases refer to the global average, which we discuss in the following section.) To figure out the effects of temperature increases, scientists depend on sophisticated models. Not models that you build when you're a kid by using papier mâché — these kinds of models are mathematical, designed to be run on a computer, and simulate the functioning of the Earth's atmosphere and climate. (See the "How climate models work" sidebar, in this chapter, for more information.) Researchers input data about the climate and how it works, and then start modifying that data to create various alternative scenarios.

How climate models work

The climate is affected by both the atmosphere (the part that everyone talks about the most) and the oceans. Changes in the air happen quickly, and changes in the oceans happen very slowly. So, scientists have been able to study air changes relatively easily, but they have quite literally had to wait and see what happens to the oceans. And because the ocean actually affects the bulk of the climate, they're also having to wait and see what happens to the entire climate. So, scientists need climate models, projected scenarios created by super computers, to help predict major climate changes.

The most complex climate models, such as those used at NASA, look at the Earth in three dimensions. The computer divides the atmosphere and oceans into square columns. Each of these columns has its own set of weather information based on the history and current status of the area. This information gives the computer a base to work from. Then, the researcher running the model changes the numbers to see what would happen if one condition changed, such as air temperature. For short-term projections (looking forward a day to a month), an advanced computer can make the calculation in 20 minutes. But making longer-term projections (such as 50 years from now) can take a month or two. A global circulation climate model can take as long as a year to produce results after researchers input all the variables.

Scientists figured out, for example, how hot the climate would need to become to melt the entire world's ice sheets — this melt would cause sea levels to rise, which would flood coastal cities around the world. At the same time, the scientists figured out the amount of greenhouse gases needed to reach these temperatures.

By looking at these different possibilities, scientists could tell which effects of climate change humans can deal with and which ones are beyond humanity's ability to adapt to or control.

The IPCC defined an average global temperature rise of about 3.6 degrees Fahrenheit (2 degrees Celsius) above 1850 levels as the official climate change warning zone, but that temperature increase is below the point of no return, or the tipping point. If the temperature goes to 5.4 degrees Fahrenheit (3 degrees Celsius), 7.2 degrees Fahrenheit (4 degrees Celsius) becomes inevitable. At 7.2 degrees Fahrenheit (4 degrees Celsius), 9 degrees Fahrenheit (5 degrees Celsius) becomes inevitable. And so on. The increases soon outstrip any human ability to slow or control those increases. The increased warming becomes inevitable because of positive feedback loops (see Chapter 7 for the lowdown on feedback loops). Melting permafrost releases methane, speeding more warming. Melting icecaps reveal more dark water, speeding warmer ocean temperatures and more ice melt. Dryer conditions lead to more forest fires, releasing more carbon and causing more warming. This domino effect could lead to an unlivable world.

No one knows exactly where that tipping point for global warming is. Scientists know only that humanity has a chance to avoid it by holding carbon dioxide concentrations to no more than 450 ppm, to keep the planet's average temperature increase at or below 3.6 degrees Fahrenheit (2 degrees Celsius). The IPCC says that the average global temperature will rise by 3.6 degrees Fahrenheit (2 degrees Celsius) when total carbon dioxide levels reach 425 to 450 ppm. Trouble is, it's moving upwards at 2 ppm per year and is currently at about 385 ppm of carbon dioxide as we type this.

Uncertainty always exists when it comes to making predictions. Acknowledging this, the IPCC actually hedged its bets slightly and said that the warning zone is between 2.7 and 4.5 degrees Fahrenheit (1.5 and 2.5 degrees Celsius) above 1850 levels — or 1.3 and 3.1 degrees Fahrenheit (0.7 and 1.7 degrees Celsius) above current levels. Its assessment report suggested that people consider the critical warning zone to be 2.7 degrees Fahrenheit (1.5 degrees Celsius) above 1850 levels — or 1.3 degrees F (0.7 degrees C) above current levels — on the basis that it's better to be safe than sorry. (The United Nations calls this philosophy the precautionary principle.)

In fact, humans have already committed the planet to a 2.9-degree Fahrenheit (1.6-degree Celsius) temperature rise versus 1850 levels — or 1.4 degrees Fahrenheit (0.8 degree Celsius) beyond today's temperature — by the end of the century, based on current climate models. Humanity could level off this increase if it implements the necessary solutions by 2010.