What Does a World at 400 Parts Per Million CO2 Look Like Long-Term?

In the give and take of the current global warming debate, it’s easy to lose track of context. Thankfully, we have a geological history to use as a window to our past. And by using that window we can see what the world will look like if CO2 levels stay where they are for long periods of time. In this first exploration, we’ll look at current CO2 levels — around 400 parts per million to give a decent idea of how the world will change if we don’t undertake the challenge of reducing these high levels of greenhouse gasses.

When Was the Last Time CO2 Levels Were This High?

It is important to note that relatively small changes in CO2 can lead to ample warming. During the last ice age, more than 10,000 years ago, CO2 levels were stable in a range between 180 and 210 parts per million. At the ice age’s cessation, CO2 levels rose to 280 parts per million. This relatively small rise of about 70 parts per million had dramatic consequences. Temperatures rose by about 5.5 degrees Celcius (10 degrees Fahrenheit).

Today, industrial activity and fossil fuel consumption has resulted in nearly 120 parts per million of additional CO2 added to the atmosphere. This addition has occurred over a very short time-scale when compared to past changes in CO2 levels and additions of 2-3 parts per million continue each year.

However, assuming CO2 were to stabilize. Assuming that, somehow, the world is able to reign in emissions enough to keep CO2 levels steady at 400 parts per million, what would happen?

As mentioned above, geological history gives us a basic notion. Long ago, about 3 million years ago, CO2 levels were steady in a range of 365-410 parts per million. This geological era was called the Pliocene.

What Did the Pliocene Look Like?

What would seem like a rather small difference in CO2 levels had dramatic effects. The first was that sea levels were 75 feet higher than they are currently today. The second was that average temperatures around the world were 3-4 degrees Celsius warmer (5.4-7.2 degrees Fahrenheit). Greenland and the West Antarctic Ice Sheet were ice free at CO2 levels of 400 parts per million and temperatures 3 degrees Celsius warmer than today. Arctic temperatures were much warmer — 8-16 degrees Celsius warmer than today.

This is the kind of world we can expect if CO2 levels are sustained at 400 parts per million.

Why Do Climate Models Under-predict Sea Level and Temperature Rise?

These historic temperature increases are much greater than those predicted by current climate models. The reason is that these models have not been able to take into account all the feedbacks to CO2 forcing that are intrinsic to the climate system. Models, by their nature, are simplifications and are only as good as the data that goes into them. But looking at geological history, it is quite clear that current climate models underestimate temperature and sea level rise given current levels of CO2.

How Fast Will Climate Change at a Constant 400 Parts Per Million CO2?

If, somehow, the world were able to stabilize CO2 at 400 parts per million, how fast would the world see 75 foot sea levels and 3-4 degree Celsius temperature increases? In short, this is the one million dollar question. Fossil fuel special interests would like us to believe that these changes would be gradual and slow to happen. In fact, many fossil fuel interests would have us believe that climate change isn’t happening at all, or, if it is, that its impacts will be far milder than the geological record would indicate. Sadly, the fossil fuel companies are misguiding themselves and the rest of us for their own short-term economic gain.

Paleoclimate data points to rapid, non-linear, responses to increases in CO2 levels. In some cases, temperatures have rebalanced over the course of decades and normally during periods of centuries or less. In some of the most radical cases, the changes have occurred on time scales measuring as few as ten years. Given the rapid rise of CO2 to its current state and likely feedbacks to result, we could expect to see a majority of that 75 feet in 300-600 years. That means severe consequences could ramp up before the end of this century pushing sea levels by ten to fifteen feet or more. You won’t see the IPCC posting a report that makes this kind of a statement, but it certainly is a potential, even if CO2 levels stabilize at ‘only’ 400 parts per million.

Most likely, current predictions of 1-2 meters of sea level rise by the end of this century is still a conservative forecast even for what would happen in a world where CO2 levels remain stable at 400 ppm. Even at constant CO2 levels of 400 ppm, we are looking at sea level rises in the range of 1.5-4.6 meters per century or more.

Business As Usual Estimates Place CO2 at Around 1000 Parts Per Million By the End of This Century; What Would That World Look Like?

Unfortunately, the world has yet to adopt serious policies that curtail greenhouse gas emission or reduce the level of CO2 in the atmosphere. And, even more concerning, world carbon sinks are beginning to contribute their own greenhouse gasses to the world climate system. Unless very rapid emissions reduction regimes are put into place, the world of the Pliocene, as strange and radically different as it may seem, will look like paradise compared to a world that reaches 600, 800, 0r 1000 parts per million CO2. And it is this increasing likelihood that we will explore in another blog.