Hurricane Sandy fizzled out over Quebec Wednesday morning, leaving a trail of devastation along the US East Coast and into southern Ontario. As I write this, Sandy’s death toll stands at 132 people – 71 in the Caribbean and 61 in the United States. Since making landfall in the US, it flooded the New York City subway system, left 8 million people without electricity (6.5 million of whom still lack it), destroyed the Atlantic City boardwalk, and shut down the New York Stock Exchange for 2 days. Estimates of its economic damage are up to $50 billion, making it the 2nd most expensive storm in US history, after Hurricane Katrina, which devastated New Orleans.
Figure - Hurricane Sandy seen Monday morning as it strikes the US. Image courtesy of NASA.
Sandy is a taste of things to come. As our planet warms – due to our activity – hurricane intensity will rise. Indeed, it’s rising already.
Hurricanes are giant heat engines. While over the sea – where all hurricanes are born and strengthen, and where most die out without ever touching land – they gain intensity from warm sea surface temperatures. Sandy grew so strong in large part because the ocean surface off the US East Coast this summer was 5 degrees Fahrenheit warmer than the norm. That high temperature drives more evaporation of sea water into the air above it, and more direct energy transfer into that air, adding power to the moist, destructive cyclone that is a hurricane.
Hurricane modelers have long understood this. And so it’s no surprise that hurricane models that take into account changing climate predict more total energy in future hurricanes as the planet warms. To be clear, models of the US eastern seaboard are divided on whether we should expect more hurricanes. Some predict more, some predict the same number, and some predict fewer. But a common factor is that hurricane models almost universally expect more total hurricane power - stronger winds, longer lifetimes, larger storms. That hurricane power – the total energy of hurricanes – is their total ability to destroy.
Modeling weather phenomena is incredibly challenging. We should trust models in proportion to how well they predict actual reality. To bolster our confidence in these hurricane predictions, then, we can look at how hurricane activity has changed as the planet has warmed. That historical data validates what the models say. Since the beginning of the 20th century, the number of “major hurricanes” in the North Atlantic has more than doubled.
Figure - As the planet has warmed, the number of tropical storms, hurricanes, and major hurricanes in the North Atlantic has risen. Image courtesy of GlobalWarmingArt.com
And the total destructive energy of hurricanes has risen since the 1970s, in a way that correlates extremely well with sea surface temperatures. Climate scientist Kerry Emanuel used records of North Atlantic hurricanes and North Atlantic sea surface temperatures to study how the two relate. What he found, shown in the graph below, is that the two move almost in lock step.
From 1950 to 1970, North Atlantic sea surface temperatures cooled slightly, by about 0.4 degrees Celsius. Total hurricane power in the North Atlantic dropped with it. (Hurricane power below is shown as “PDI” – the Power Dissipation Index, which measures the total energy in all hurricanes in the area based on their wind speeds and area.) From 1970 through 2005, sea surface temperatures rose by around 0.8 degrees Celsius (double the amount they’d fallen in the previous decades), and total hurricane power increased with them. Other researchers have since found the same correlation in temperature and storm data from other parts of the world.
Figure - Total hurricane power (green line and right axis) moves almost in lock step with sea surface temperatures (blue line and left axis). Image from Emanuel (2007).
The most sobering thing about the graph above, however, is not the way the lines move in lock step. It’s the extreme sensitivity of hurricane energy to temperature. From 1970 to 2005, sea surface temperatures rose by 0.8 degrees Celsius. That change – hardly something that seems dramatic – led to roughly a quadrupling of total hurricane energy in the area. Look again at the two scales in the graph. The right scale covers a dramatically different range than the left. Small changes in temperature lead to massive changes in hurricane intensity. With sea surface temperatures projected to continue rising, particularly in coastal regions close to land, the total energy available to tropical storms is likely to soar in the decades ahead.
The Sea Ice Connection
One more climate factor contributed to Sandy. This year, we saw record low Arctic Sea ice – the lowest in thousands of years. What does that have to do with storms? It affects which way they turn and how long they linger over one area. Only around 1% of hurricanes in the North Atlantic ever make landfall. The vast majority are born at sea and die at sea. And very few hurricanes turn west at such high latitudes as Sandy. Generally, if a hurricane or tropical storm makes it as far north as New York City, the prevailing winds will force it out to the east and away from land. This happens, in large part, due to the arctic jet stream – a high speed band of wind that blows from west to east in the upper atmosphere, driving prevailing winds in the region.
But the jet stream gets its energy from the temperature difference between the Arctic and the regions to the south. This year, with Arctic Sea ice at an all-time low, the Arctic was quite warm (in a relative sense), reducing the temperature differential between the Arctic and the regions to its south, and lowering the speed of the jet stream. Indeed, between 1979 and 2011, the jet stream weakened by around 14%. (It was likely even weaker in 2012, but high quality numbers are not yet available.)
That weak jet stream increases the incidence of ‘blocking’ weather patterns, where a weather phenomenon lingers over an area for a prolonged period of time. The weak jet stream was likely a contributing factor to the record-breaking drought that the US saw in 2012, for instance.
And that weak jet stream also means less of a push towards the east – and away from land – for hurricanes as they head north. Sandy, it seems, got a double dose of climate – more energy from warmer waters, and less of that push away from land as a result of less sea ice in the Arctic.
The Big Picture
No single storm is the result of climate change. Storms as big or bigger than Sandy have happened in the past, and could happen, even on a cooler planet. But a warmer planet feeds more total energy to storms, and means more total destructive energy. It means more storms like Sandy, and far worse, in the years to come.
This year, in the Presidential debates, for the first time in decades, climate change was not discussed. Yet climate change is a very real phenomena and one that has real consequences – in lives taken, in infrastructure destroyed, in cities shut down for days, in (as we saw this summer) crops dying in the fields. Climate change isn’t some future phenomena, decades away, that only our great-grand-children will have to deal with. Its impacts are being felt here and now. We’ll feel them more and more intensely in the years ahead. The time to act, honestly, was years ago. But today is a better time than tomorrow.
This year we saw a triple whammy of climate linked events. Record breaking drought in the United States. Record low sea ice in the Arctic. And now a record breaking storm on the US East Coast. In his James Bond novel, Goldfinger, Ian Fleming wrote that “Once is happenstance. Twice is coincidence. Three times is enemy action.”
We’ve seen the face of the enemy. And it’s time for us to take action of our own.