Module 2: Forecasting Climate Change

Climate forecasts attempt to anticipate how the Earth’s climate will change over time. These forecasts are like weather forecasts, but they look much farther into the future—decades and even centuries ahead.

Climate forecasts are based on “global climate models” (GCMs) that are based on the physical laws governing the climate as we understand them. GCM’s have become more and more sophisticated over time, largely because computing advancements have made it possible for GCMs to get a lot more detailed in their representations of the how the climate works.

GCMs today do a good job of recreating the Earth’s climate as it has changed (due to natural drivers) over millions and billions of years. One reason for that is that the models are calibrated against the Earth’s climate history. If a model were to perform very poorly in recreating past climates, then its builders would probably try and figure out what it was doing wrong, and “fix it.”

The fact that the Earth’s climate has changed many times during Earth’s history doesn’t mean that those changes were uneventful. According to the EARTH’S PRECARIOUS HISTORY Mini-Course, for example, life on the planet has come close to being extinguished multiple time. The Mini-Course is based on Peter Brannen’s fascinating 2017 book The Ends of the World - Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions. It is a great overview of the Earth’s history, and does a great job of analogizing from Earth’s history to today’s human-induced climate change. He concludes that Earth’s history suggests that “forcing” atmospheric change at a rate at least 10x faster than has ever occurred “naturally” is a bad idea, and is analogous to playing Russian Roulette with the climate.

Climate models consistently show that if human activities continue to release large amounts of greenhouse gases to the atmosphere, the planet will continue to warm. And climate models have accurately anticipated many of the changes we've seen so far. For example, global climate models anticipated that the Arctic would warm much faster than other parts of the world, and that's exactly what has happened.

Why There’s Uncertainty in Climate Forecasting

While climate scientists are confident that the Earth is warming and that human activities are the main cause, there’s still a lot we don’t know about how climate change will play out in the future. Here are some key areas where uncertainty comes into play:

Climate Sensitivity: This term refers to how much the Earth’s temperature will increase in response to a certain amount of greenhouse gases in the atmosphere. Some models suggest the temperature could rise significantly, while others predict a more moderate increase. The exact level of sensitivity is still uncertain, which means we can’t be sure how much warming to expect.
Future Greenhouse Gas Emissions: Another major uncertainty is how much greenhouse gas we’ll emit in the future. This depends on factors like economic growth, technological advances, and government policies. If we reduce emissions significantly, we can slow down climate change. But if emissions continue to rise, the impacts will be much worse.
Future Policies and Actions: What governments, businesses, and individuals do to address climate change will also affect future outcomes. Strong policies to reduce emissions and adapt to changes can make a big difference, but there’s uncertainty about how quickly these measures will be implemented and how effective they’ll be.
Feedback Loops: Climate systems are full of feedback loops—processes that can amplify the effects of climate change. For example, as ice melts, it exposes darker ocean water, which absorbs more heat and causes more ice to melt. These feedback loops are difficult to predict, adding another layer of uncertainty.

Why Uncertainty Equals Risk

Uncertainty doesn’t mean we should ignore climate forecasts. In fact, it’s a reason to be even more concerned. The uncertainty means that things could turn out worse than we expect, making it essential to take action now.

The risks of not acting include more frequent and severe weather events, loss of biodiversity, and significant impacts on human health and economies. Even though we don’t know exactly how things will unfold, we know enough to understand that the risks are real and that the consequences could be severe.

Until now the most convenient way to explore the implications of climate change via “temperature maps” was via the Probable Futures website. Their maps allow you to very easily explore a number of climate variables, mostly related to heat, precipitation, and dryness, for up to 3^o C of average global temperature (AGT) increase (in 0.5^o C increments). The maps are great and colorful and ultra-easy to use. But they’re also quite limited, only extending to 3^o C, and focusing on “what happens if AGT changes by “X,” as opposed to how likely is it that AGT will change by “X”? That’s where the new Copernicus Climate Atlas comes in.

The results of global climate models will vary dramatically based on the timeline involved. The longer the timeline, the more dramatic the likely change. For example:

Short-Term Changes (Next 10-20 Years): In the near future, we can expect to see more frequent and intense heatwaves, shifts in rainfall patterns, and more powerful storms. These changes are already beginning to affect how we live and work. That said, the short-term is also characterized by natural variability that makes it harder to differentiate between natural variability and the climate change signal.

Medium-Term Changes (Mid-Century, Around 2050): By the middle of this century, the impacts of climate change will become more severe. Sea levels will rise, flooding coastal areas, and more regions will experience regular droughts, causing more displacement of people and strain on resources. Over longer periods like this, the “climate signal” will become more obvious against “natural variability.”

Long-Term Changes (End of the Century and Beyond): If we don’t take significant action to reduce greenhouse gas emissions, the end of this century (and beyond) could see even more drastic changes. The “climate signal” will clearly dominate “natural variability” when it comes to thewidespread loss of ice sheets, large-scale extinction of species, and severe disruption to global weather patterns. Remember that historically, each 1^o C in average global temperature increase has been associated with ~20 meters of sea level rise around the world. We haven’t seen anything like ~20 meters of sea level rise in recent time, even though average global temperature has already increased by more than 1^o C. But over time, sea level rise is likely to “catch up.” The question is, over how long a period?

When it comes to forecasting climate change, there are all kinds of great resources available to you. One of the easiest to take advantage of is the Probable Futures website, which let’s you instantly see how the world is likely to change per 0.5^o C of average global temperature increase, out to 3^o C (unfortunately the website doesn’t extend to 4^o or 5^o C, despite the plausibility of those two outcomes). You can instantly explore include Global Hottest Web-Bulb Days Days Above 32^o C Frequency of Today’s 1-in-100-year Storms Wildfire Danger Days Annual Precipitation Freezing Days Snowy Days Nighttime Temperature Likelihood of Year-Plus Drought and Change in Climate Zones. If you play around with these variables, you’ll get a good sense of what current global climate models are suggesting we should be prepared for.

While the Probable Futures website is excellent, there is now another resource that allows anyone to become a climate forecaster. It’s the European Copernicus website, based on the most recent CMIP-6 “family” of global climate models, that allows you to explore literally hundreds of potential climate scenarios.