Global Warming and Predictions of an Impending Ice Age – Predicting Future Climate

This is the fourth and final post in a series on the connection between the sun, sunspots, and climate.

Where is the climate headed? The Intergovernmental Panel on Climate Change (IPCC) predicts continued warming [pdf]. But others proclaim that an ice age is just around the corner (like this editorial from the online paper the Australian). Who’s right?

Predicting the future is a bear. Predicting climate change is especially uncertain. But given global warming's potential consequences, we ignore an educated look ahead at our own peril. And let's face it, models serve us well in all sorts of areas in the modern world.

Predictions of Warming

The IPCC's warming predictions are based on climate simulations with an important underlying assumption -- what I call the “all things being equal” assumption: the models assume that external factors, such as the solar output, continue to behave pretty much the way they have in the recent past. These models also use a reasonable range of projections for greenhouse gas emissions and values for uncertain model parameterization. All the models predict continued and significant warming.

Of course, these predictions could all be wrong. The models themselves could have a serious flaw (which causes them to over-predict rather than under-predict greenhouse warming), and/or some important external parameter like the Sun could suddenly, unexpectedly change in a way to cancel out greenhouse warming. But while it's important to recognize these possibilities, it is essential that we weigh their probability versus the risks we face if the models are correct. (And remember the models could also be under-predicting.)

In that vein, let’s look at two of the more popular arguments that global warming is not a problem.

Ice Ages

Our current climate regime -– a regime we’ve had for the past ~2 million years –- is characterized by long periods of ice ages and shorter warm periods. The last ice age ended about 12,000 years ago, and since then we've been in a warm period. Some argue that global warming is no concern, since the Earth will naturally switch back to an ice age.

This is very likely to be true: an ice age is almost certainly in our planet’s future. But it’s a question of when.

Our current concerns about climate change focus on the coming decades to the next century –- the time period relevant to our children's and grandchildren's experience. But the ice age/warm period cycle operates on a time scale of tens of thousands of years.

Scientists have figured out that ice ages are triggered by subtle changes in the Earth’s orbit about the Sun. The next such triggering is not expected to occur any time soon – tens of thousands of years from now. Not quite soon enough to be relevant to our children's well-being.

The Sun as the Deux Ex Machina Saving Us from Global Warming

Others argue that the Sun’s output will suddenly shut down and stop global warming. Indeed, some now predict that a brutally cold period much like the Little Ice Age that began in the 1200s and ended in the mid-1800s has already begun. There are two intriguing aspects to this claim:

1. The Little Ice Age was likely caused, at least in part, by a relatively quiescent Sun. Characteristic of this quiescence was the Maunder Minimum (~1645-1715) when there was an anomalous absence of observed sunspots (see my earlier post).
2. We are currently in an unusually long and strong solar minimum. In fact, 2008 has been a year of unusually low solar activity, with more days sans sunspots (called “spotless days”) than any year in the past five decades and on a pace to challenge 1913 as the century's most quiescent year (see graphic).

Top: A histogram showing the last half-century's blankest years. The vertical axis is a count of spotless days in each year. The bar for 2008, which was updated on Sept. 27, 2008, is still growing. Bottom: A histogram showing the last century's blankest years. As in the above histogram, the bar for 2008 is still growing. (Credit: NASA)

Some now argue that the current quiescent period is heralding a new Maunder-like minimum in the Sun and, as a result, a new ice age. Don’t worry about global warming, they advise -- worry about global cooling.

Think (and Grok) Before You Leap to Conclusions

Of course, proving such a prediction wrong is impossible, but please keep a number of things in mind before you run out to buy that snow parka:

1. Using one year’s worth of sunspot data to infer a long-term climatic trend is a highly questionable practice.
2. While the Sun's current quiescent period is unusual, it is not unprecedented. The year 1913 had more than 300 spotless days, and an ice age did not follow. Indeed, the long-term trend in global warming continued apace.
3. Sunspots affect climate, but they are not the only factor. Don’t forget that 2007, the third most spotless year of the past 50 (see graphic), was also the third warmest year on record. And while the winter of 2008 was unusually cold (possibly because of La Nina), the rest of the year has been quite warm. According to the National Oceanic and Atmospheric Administration, March 2008 was the second warmest March on record; the spring was the eighth warmest on record; and the summer was the ninth warmest. And bear in mind that these relatively high temperatures occurred while the Sun was at its minimum and a strong La Nina persisted in the South Pacific.

Predicting future climate is an “imperfect science.” Uncertainties and unknowns are unavoidable. Still, our models represent our best understanding of how the climate works. Does that mean that the Sun couldn’t shut down and “save us” from global warming? Of course not. Would I be willing to bet my children’s and grandchildren’s future on that possibility? No.

Other Posts in Global Warming and Predictions of an Impending Ice Age

Part 1: Total Solar Irradiance

Part 2: Sunspots

Part 3: Global Warming Since 1998