Global Warming

The Earth is getting warmer, and the consensus of scientific opinion is that there is at least a 95% probability that human activity (burning fossil fuels, deforestation) is to blame. Maybe we should hope they’re right.

You see, there is a thing called an ice age. Between ice ages, Earth’s polar regions are free of ice. During an ice age, or more precisely a glacial age, Earth’s surface cools and ice forms at the poles and advances toward the equator. There is geological evidence that strongly suggests that Snowball Earth events have occurred during one or more ice ages. During a Snowball Earth event, Earth’s entire surface, including its oceans, is covered by ice.

Ice ages last a long time. The most severe ice age of the last billion years was the Cryogenian which occurred from 850 to 630 million years ago and may have produced a Snowball Earth. However, only single-cell bacteria lived on Earth at that time. The current ice age, called the Pliocene-Quaternary glaciation, started 2.58 million years ago. Since then, ice sheets have advanced and retreated on 40,000 to 100,000 year time scales.

Within an ice age, there are colder periods in which ice sheets advance. These colder periods are called glacials or glacial advances. Also within an ice age are warmer periods in which ice sheets retreat. These warmer periods are called interglacials or glacial retreats. Glacial advances last a long time, whereas interglacials are shorter. The last glacial period ended more than 11,000 years ago. The Earth is currently in an interglacial period called the Holocene. How long will the Holocene last? No one knows for certain. We know the last interglacial (prior to the current one) lasted for 12,000 years.

It was the end of the last glacial advance and the advent of the current interglacial that allowed human civilization to arise and flourish. Agriculture became possible, and with it civilizations arose. Civilizations gave birth to art, religion, science, writing, mathematics, technology, industry: almost everything we have today. It’s all possible because of the long interglacial warm period in which we happen to be living.

When the current interglacial ends, it could mean the end of civilization, or it could mean the end of us. Humankind would probably not survive a Snowball Earth event. But even a less severe glacial advance would severely strain our ability to survive. Supplies of fresh water would freeze solid, crops would fail, livestock would die. Humanity might be thrown back to a Stone Age existence.  During the most recent glacial advance, the area of the United States now known as New York City was covered by a one mile thick sheet of ice. Clearly, another glacial advance is something we want to avoid.

However, humankind is not without hope. If human activity can influence the climate, and if we choose to act wisely, we may be able to avoid another glacial advance. If we can warm the Earth by our own activity, then we’re safe, are we not? We won’t freeze.

But hold on. Some climate scientists think that if polar ice melts too quickly it may generate enough fresh water to disrupt the Gulf Stream. The Gulf Stream carries warm water from the Gulf of Mexico to Europe, keeping northwestern Europe temperate. Disrupting the Gulf Stream would throw northwestern Europe into a deep freeze. If other ocean currents were also disrupted, it could trigger a worldwide glacial advance.

Another scenario is a runaway greenhouse effect. In this event, a positive feedback between Earth’s surface temperature and atmospheric opacity increases the strength of the greenhouse effect until the oceans boil away. This is believed to have happened in the early history of Venus. The IPCC says there is virtually no chance of this event being induced on Earth by human activity. So we’re safe from that – unless the IPCC is wrong.

If we tip the Earth’s climate out of its present ice age, the polar regions will become ice free. An ice free Earth will be a catastrophe for many humans. Sea levels would rise more than 200 feet, drowning coastal areas and low-lying islands – and the entire state of Florida. But it seems to be the kind of catastrophe that the human race and civilization could probably survive. The world would be different, of course, but we could still grow food, have access to fresh water, and live in cities – things that are difficult to do under a mile of ice.

So here is where things stand: we are living in an interglacial warm period which, at some point, will likely end. We need to avoid another glacial advance, which means it is desirable that we are able to warm the planet’s surface, but not so fast that we inadvertently tip the climate into another glacial advance, nor so warm that it triggers a runaway greenhouse effect, assuming such an outcome is possible. To do that, we need to know what human activities cause global warming and precisely how much impact those activities have on Earth’s climate. We need to know how much warming is enough, how much is too much, and how fast is too fast.

All things considered, I’m glad there are smart people working diligently to find answers to our climate questions, and I’m prepared to listen and give careful thought to what they have to say.

As for the scoffers, let me put it to you a different way. Suppose you were preparing to drive a heavy vehicle across a small wooden bridge, and then you discovered that the majority of bridge engineers were predicting the bridge would collapse under you, ending your life. Would you, without doing any math, without doing any computer modeling, without knowing anything about the science of bridge design, simply say, “Nah. I don’t believe it. I’m going.”? Because if that is your response, it means you’re in denial, and that is not a good place to be, whether it’s your own personal future or humanity’s future that is on the line.