Extreme Weather Autopsies and The Forty-Year Forecast - Your Weather is Your Climate - The Weather of the Future: Heat Waves, Extreme Storms, and Other Scenes from a Climate-Changed Planet - Heidi Cullen

The Weather of the Future: Heat Waves, Extreme Storms, and Other Scenes from a Climate-Changed Planet - Heidi Cullen (2010)

Part I. Your Weather is Your Climate

Chapter 4. Extreme Weather Autopsies and The Forty-Year Forecast

I like watching basketball, but I’ll admit that most of the time I can’t keep up with it. Until the perfect moment when someone flies through the air and makes a basket, I can’t see a damn thing. To me, the game is a lot of noise with very little signal. I’m much better off if I can watch a game with an aficionado. Watching the climate is no different. Learning how to see the climate system in play is like learning how to see the Lakers run a screen or watching a northeaster swing down from Canada. This is an art and a science, and it takes a trained ear to hear through the noise. Sometimes, hearing through the noise requires help in the form of a slow-motion instant replay. This is especially true when scientists attempt to understand the connection between global warming and weather that’s happening to us right now.

The weather isn’t what it used to be. In fact, all the data we’ve collected over the past fifty years point to the fact that the weather is getting more extreme.1 But trying to isolate the fingerprint of global warming within the weather is much harder than isolating the fingerprint of global warming within the climate system. That doesn’t mean it’s not there; it just means that discerning climate change in the weather is a much noisier, more chaotic, and more complicated process. Ultimately, as in sports, the statistics can help us find the story buried beneath the noise. And climate scientists have come up with some very clever variations on using a slow-motion instant replay of the weather to help them understand how the statistics of extreme events are changing.

It turns out that you can use climate models as an instant replay to re-create a specific weather event. Think of this as like an autopsy, except that it’s being performed on a specific extreme weather event. And although it can’t determine the individual cause of the weather event, it can allow scientists to calculate the odds of such an event. Those odds can speak volumes if you know how to read them. The models can measure how much global warming shifted the odds in favor of allowing a specific type of weather event to happen. And, perhaps even more strikingly, the models allow us to see how those odds will play out and change in the future.

The European heat wave of 2003, an extreme weather event that killed more than 35,000 people, offers the best example of how climate models can help us see the global warming embedded within our weather. Public health officials were shocked at the scale of the human casualties caused by the heat. The largest number of casualties was in France, where almost 15,000 people perished in the first three weeks of August. Climate scientists were equally shocked at how far outside the range of historical temperature the heat wave registered. The summer of 2003 has been described as the biggest natural disaster in Europe on record.

The heat wave was dramatic. Temperatures in France soared to 104°F and remained unusually high for two weeks. There were extensive forest fires in Portugal, burning an estimated 1,500 square miles. Melting glaciers in the Alps caused avalanches and flash floods in Switzerland.


Each vertical line represents the mean summer temperature for a single year for the European region that spans [10W to 40E, 30N to 50N] over the period 1901 through 2003. Extreme values from the years 1912,1999, and 2003 are identified. SOURCE: ADAPTED FROM SCHÄR ET AL., 2004, BY C. TEBALDI, CLIMATE CENTRAL.

When we step back and compare the summer of 2003 with past summers, the picture becomes even more obvious. As you can see in the accompanying figure, there are a series of vertical lines that look rather like a bar code.2 Each vertical line represents the mean summer temperature for a single year from the average of a region over Europe that spans [10W to 40E, 30N to 50N] over the period 1901 through 2003. Until the summer of 2003, the years 1912 and 1999 stood out at the edges as the most extreme temperatures in terms of hot and cold summers. Climate scientists estimate that the summer of 2003 was probably the hottest in Europe since at least a.d. 1500.

If climate is what you expect and weather is what you get, then the summer of 2003 was far outside what anyone would have expected. Statistically, in a natural climate system with no man-made CO2emissions, the chance of getting a summer as hot as 2003 would have been about once every thousand years, or one in 1,000.

The point of this weather autopsy isn’t so much that the 2003 heat wave was, or was not, caused solely by global warming. Indeed, almost any weather event can occur on its own by chance in an unmodified climate. But using the climate models, it is possible to work out how much human activities may have increased the risk of the occurrence of such a heat wave. It’s like smoking and lung cancer. People who don’t smoke can still get the disease, but smoking one pack of cigarettes a day for twenty years increases your risk of developing lung cancer twentyfold. Thanks to some sophisticated climate models and well-honed statistical techniques, scientists can identify the push that global warming is giving the weather.

Step one was to re-create the 2003 heat wave in a high-resolution climate model using data observed during the heat wave. Scientists set up two sets of climate model experiments. One simulation included human-induced greenhouse gas emissions; the other simulation didn’t include them. Like other climate model experiments, this one essentially created two worlds: a world with human influences and a world without them. By comparing the two, the scientists could look at what the risk of having a very hot summer is now and compare that with what the risk would have been if there hadn’t been any human-influenced climate change. The difference between these two sets of odds would tell them just how much of a role humans played.

This weather autopsy, published in the science journal Nature,3 showed that human influences had at least doubled the very rare chance of summers as hot as the one Europe experienced in 2003. The climate models showed that greenhouse gas emissions had contributed to an increase in such summers, from one in 1,000 years to at least one in 500 years and possibly one in 250 years.

What is perhaps most shocking is what happens when we run the models in forecast mode instead of autopsy mode. If the summer of 2003 had been a freak of nature, we could just chalk it up to chance. But the latest climate models paint a very bleak picture. According to their predictions, by the 2040s such summers will be happening every other year. And by the end of this century, people will look back wistfully to 2003 as a time when summers were much colder. Hindsight, as they say, is twenty-twenty.

In the United States, average annual temperature has risen more than 2°F during the past fifty years, and the temperature will continue to rise, depending on the amount of heat-trapping gases we emit globally.4Along with the general increase in average annual temperature, most of North America is experiencing more un- usually hot days and nights, as well as heat waves.5 Across the United States, ever since the record hot year of 1998, six of the last ten years (1998-2007) have had annual average temperatures that have made the record books, and these six years have ranked in the hottest 10 percent of all years since 1895 (when record keeping started). Also, the United States has seen fewer extremely cold days during the last few decades. In fact, the last ten years have brought fewer severe cold snaps than any other ten-year period since records began to be kept in 1895. There has been a decrease in frost days and a lengthening of the frost-free season over the past century as well.

By 2050, mid-range emissions scenarios predict that a day so hot that it is currently experienced only once every twenty years would occur every three years over much of the continental United States. By the end of the century, such a day would occur every other year, or more often. As for the cold, we’ll be seeing even less of it. By 2100, the number of frost days averaged across North America is projected to decrease by one month; and decreases of more than two months are projected in some places.

The extreme weather of climate change is not limited to heat waves; the climate models suggest that other forms of extreme weather are also expected to increase. A warmer climate increases evaporation of water from land and oceans, and it allows more moisture to be held in the atmosphere. In other words, as the air gets warmer, it can hold more water vapor. Coupled with other warming-related changes, this additional moisture-holding capacity increases evaporation and will result in longer and more severe droughts in some areas and more flooding in others.

These trends are already beginning to be seen in the United States, and depending on where you are, you may have experienced them yourself. In the Northeast, the Midwest, and Alaska, the additional atmospheric moisture has contributed to more overall precipitation. In the West and Southwest, the opposite is happening, as those areas have seen reductions in precipitation and increases in drought.

A warmer climate also means that it rains harder when it does rain. Diagnostic analyses have shown that with higher temperatures, a greater proportion of total precipitation comes from heavy precipitation events, such as blizzards and rainstorms. Heavy precipitation events averaged over North America have increased during the past fifty years, keeping pace with the increases in atmospheric water vapor that come from higher man-made carbon emissions.

Rain is just the start. The extreme weather produced by a warmer climate might include hurricanes. Because global warming results in warmer oceans, most experts on hurricanes agree that this warming of the ocean waters will make hurricanes more powerful. Hurricanes derive their energy from the evaporation of seawater, and water vapor evaporates more easily when it’s warmer. The data suggest an increase in the number of more intense hurricanes in the North Atlantic during the past few decades. But more data are needed before scientists can be certain. Most climate models do predict that the strongest tropical cyclones will get stronger as global warming continues, but some models suggest that the total number of storms may actually decrease. There are several factors that influence the formation of tropical cyclones, including wind patterns, ocean currents, and local weather conditions. Any one of these factors might change in a warming world, in ways scientists are not yet able to predict.

But even if the jury is still out regarding the specific future of hurricanes, everyone is certain that damage will get worse. This will be due partly to population increases along the coast, and partly to the fact that global warming melts the ice caps at the poles, thereby raising the sea level. With a higher sea level come higher storm surges and more damage to our coastlines.

Ultimately, these extreme weather autopsies confirm something that many of us have long suspected: the weather is getting more extreme. The conditions have arisen for more major storms, longer droughts, and serious flooding, and they are getting worse.

These predictions and our seeming inability to heed their warming is a potential tragedy, reminiscent of Greek tragedy. Climate scientists seem to have become Cassandras, though the questions remain whether our forecasts will be heeded, and whether the harrowing scenarios of life on a warmer planet will come to pass.

The latter question is what Part II of this book is about. It explores the Achilles’ heel of seven locations around the world, looking at how climate change will remind each one of its own frustrating vulnerabilities time after time if we humans continue to emit carbon at our current rates. Part II is about the unique perspective that each location offers on the risks of climate change to humankind. It’s about using all the predictive tools that we’ve discussed—climate models, weather models, hind-casting, and extreme weather autopsies—to discuss the climate and weather scenarios for these various locations if global warming is allowed to continue unabated.

Just as the landscape of Earth is diverse and complex, so are the stories that specific landscapes will tell as climate change takes hold. These stories are the heart of Part II. Each chapter in Part II consists of two sections.

In the first section of each chapter, I’ve used climate models, environmental data, and—most important—the brightest scientific minds studying the climate in each location to help calculate and describe the specific risks each place will encounter over the coming decades. Despite all that math, models, and physics can tell us, these scientists are our most valuable tool for understanding the specific risks associated with each location. Because the climate of a place is so closely intertwined with the people who live there, it’s nearly impossible to make an accurate prediction about the future without first understanding its most important initial condition: the human population. Along with all their measurements and readings, it is this understanding of the local population that these scientists strive for on a daily basis; the people’s stories and experiences, what they’ve witnessed firsthand as they look climate change in the face every day, are as much a part of the predictions as the model-driven data. It is only by understanding the people who call each place home that we’ll be able to predict how they’ll react when climate change exposes their home to ever-increasing risks.

The second section of each chapter in Part II contains a series of predictions, based on a collection of climate models, which, taken together, offer a window into what the next forty years will look like for the place discussed in the chapter. The forecasts are works of fiction. I can’t say whether the future will play out as I’ve described, but what I can say is that these predictions are based on the best available science derived from some of the cutting-edge climate models and real weather events from the historical record. In writing them, I have pored over reams of data with the aim of turning those numbers into stories, stories that give new visual guides to what climate change might look like in a place near you.

Each forecast begins with a glimpse of the regional climate during January and July based on two possible emissions scenarios. The scenarios are drawn from an average of at least fifteen different climate models, and each scenario makes different assumptions about future human activity—including greenhouse gas pollution, land-use alteration, technological development, and future economic development.

The first scenario is based on medium-high emissions. This scenario projects continuous population growth and uneven economic and technological growth. In it, the income gap between currently industrialized and developing parts of the world does not narrow. Heat-trapping emissions increase through the twenty-first century, and atmospheric CO2 concentration approximately triples, relative to preindustrial levels, by 2100.

The second scenario is based on lower emissions. It characterizes a world with high economic growth and a global population that peaks by mid-century and then declines. There is a rapid shift toward less fossil fuel-intensive industries and the introduction of clean and resource-efficient technologies. Heat-trapping emissions peak at about mid-century and then decline. Atmospheric CO2 concentration approximately doubles, relative to preindustrial levels, by 2100.

Some of these predictions have geopolitical implications; others have simply national ramifications. But one thing that’s certain is that none of these scenarios will be happening in a vacuum. In a climate-changed Earth, every inch of land, ocean, and air will be affected. These stories represent some of the most dramatic and vulnerable locations, but they also represent places where the human species has been living for millennia, including some that will be rendered inhospitable by the changing climate. With regard to climate change, the worst-case scenario can be prevented through infrastructure investment (adaptation) and the adoption of clean energy technology and emissions reductions (mitigation). Some of these predictions examine what might happen if one location decides to adapt; others examine what might happen if a location refuses to change.

Ultimately, the basic assumptions driving these forecasts are that we will continue to burn fossil fuels, that the global population will continue to grow, and that because of both factors, greenhouse gases will continue to rise. The end result of this rise is that weather will not only become worse—it will become downright awful. Indeed, as the coming pages will show, the weather of the future and the way that weather affects life on Earth will be far worse than anything we’ve seen before.

Though these prophecies, as I’ve suggested, contain the seeds of a Greek tragedy, ultimately the forecasts also contain a kernel of hope, because unlike the prophecies in Greek tragedy, they are changeable. The forecasts paint a picture of just one possible future. While these forecasts, or indeed any forecasts, make certain assumptions about how trends will continue, one true variable they cannot approximate with much accuracy is our own behavior. We are the factor that could render all these predictions false, because we alone have the power to reduce our global carbon footprint. The question that we must now answer is how. In the end, these forecasts pose a question that is vital to our collective future: if we are really capable of forecasting the future and seeing the devastation of a changing climate in advance, will we act to prevent it? Can we rally around this forty-year forecast for the good of the world, or will we wait until the levees break before we decide to act?