News•March 20, 2013

If You Thought 2012 Was Hot, Just Wait a Few Years

It’s already March, but the country is still recovering from last year’s extreme weather and climate events. As the hottest year on record in the U.S., 2012 brought intense drought across the Midwest and Great Plains, record wildfires in the West, and Hurricane Sandy’s path of destruction to the East Coast.

But if you thought 2012 was hot, just wait a few years. Within the coming decades, years like 2012 may become simply “average.” And if we keep emitting CO2 at the current rate, it won’t be long before years like 2012 feel cold.

Average U.S. temperatures are already warmer than they used to be. The 2000s were hotter than the 1990s, and the 1990s were hotter than the 1980s. What’s happening in the U.S. looks like an enhanced version of the global trend, possibly helped by the effects of some natural variations. Scientists have shown that most global warming stems from rising greenhouse gas emissions that end up in the atmosphere from burning fossil fuels. And how those emissions are managed in the coming years will dictate how much hotter it will get this century.

And as future temperatures regularly reach the record levels of 2012, we can also expect to see more of the types of climate extremes that gripped the U.S. last year, including droughts, sea level rise, and more damaging storms.

The good news? Years like 2012 — when the average U.S. temperature was 3°F higher than was typical last century — could be about as bad as it gets.

The bad news? That will only be possible with extraordinary efforts to reduce carbon emissions and significant deployment of technology that captures CO2 already in the atmosphere (which isn’t even ready for commercial use yet).

Climate Central’s interactive (above) shows that within 10-30 years, a year like 2012 will be an average year in the U.S. And a scant few decades later, typical years will probably be even hotter, perhaps by several degrees.

Climate models can predict how global temperatures will rise this century, depending on how much more CO2 is released into the atmosphere. The climate science research community has recently developed four scenarios for model simulations, each with a different greenhouse gas trajectory. Using these scenarios, it’s easy to see how radically different temperatures might be by 2100. Each line on the graph represents the average of dozens of simulations for each scenario. You can click through the buttons on the bottom of the interactive to see the full range of values for each scenario.

For example, in the interactive graphic above the high-emissions scenario (black) represents a future where greenhouse gas emissions continue to increase through 2100 and the total amount of CO2 in the atmosphere climbs rapidly. With these kinds of emissions, average annual U.S. temperatures could be more than 7°F higher than what is normal today. Scientists believe that in order to avoid the most serious consequence of climate change, global temperatures shouldn’t rise more than 3.6°F (or 2°C) above the preindustrial average (and, globally, temperatures have risen nearly half that amount already).

Average annual temperature in contiguous U.S.
Click image to enlarge. Credit:Book It: 2012, The Hottest U.S. Year on Record report.

On the other hand, the low-emissions scenario (green) is one where atmospheric greenhouse gas concentrations in 2100 are actually lower than they are today. Of course, that will only be possible if we drastically reduce our emissions and actually remove CO2 that is already in the atmosphere (which requires technology that isn’t yet viable on a large scale). Even if we took those incredible measures — and as yet there’s virtually no indication we will — temperatures are still going to rise in the next several decades and average U.S. temperatures will probably be at least as high as they were in 2012.

The other two medium-emissions scenarios (blue and red), fall somewhere in the middle. They illustrate a future where greenhouse gas emissions eventually stop increasing, but the total amount of greenhouse gases in the atmosphere continues to grow. The scenarios differ on when emissions stabilize, but the effect will be similar: rising temperatures through 2100.

So where do we stand today? Our current trajectory is close to the black high-emissions scenario. If we stay on this path without curbing emissions, we can expect to regularly see average temperatures at least 7oF higher than we do today. And with that will come more weather extremes, droughts, wildfires, sea level rise, and more destructive force from hurricanes like Sandy.

Even with action to help mitigate climate change, the interactive shows just how sobering these numbers truly are. All that CO2 we’ve already added to the atmosphere will keep temperatures climbing for several decades. Under any imaginable scenario — even in the best-case scenario — soon we’ll likely see years like 2012 much more regularly.

Methods:

In order to determine when last year’s record-breaking temperature will be that of an average year, we looked at how much climate models(2) predict average U.S. temperatures to rise this century. Using global simulations of four potential greenhouse gas emission trajectories (the Representative Concentration Pathways(1)), we’ve calculated future average global temperatures (for each trajectory). We have then estimated future average U.S. temperatures by scaling the global temperatures according to a factor calculated on the basis of the ratio of global temperatures to U.S. temperatures by the end of the 21st century, which has been shown to apply reasonably accurately to other periods during the length of the simulation, according to the method known as “pattern scaling”(2). The method is not supposed to predict exact year-by-year changes, but it is appropriate to get a sense of the long-term trends in mean temperatures at regional scales (U.S. average-scale) and thus can give us a good estimate of the time frame — on the order of a decade or so — by which temperatures in the U.S. will reach the threshold represented by this year’s anomalously warm temperature.

We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups for producing and making available their model output. For CMIP the U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals.

Special acknowledgments also to Julie Arblaster (University Corporation of Atmospheric Research and Australia’s Bureau of Meteorology), who assisted Climate Central with computations.

1 Santer, B. D., T. M. L. Wigley, M. E. Schlesinger, and J. F. B. Mitchell, 1990: Developing climate scenarios from equilibrium GCM results. Max-Planck-Institut-für-Meteorologie, 29 pp. pp.

2 Calculations were based on the model simulations from CMIP5, the most up-to-date series of climate model simulations, which were used for the upcoming IPCC 5th Assessment Report.