Climate change in the 21st Century
The Deniers
It is often claimed by those who deny the reality of climate change that scientific forecasts about the impact of global warming are far too uncertain to merit taking action. There is no reason to suffer the inconvenience of leaving the planet's fossil fuels unburned when the current analyses of meteorologists, oceanographers and geophysicists will probably turn out to be false alarms, they argue.
Such contention is dangerously false. For a start, scientists' warnings about future weather patterns are certainly not overreactions to the evidence they have gathered. In most cases, observed climate changes – the slump in summer sea ice coverage in the Arctic in recent years is a good example – have turned out to be far more drastic than researchers had originally predicted. Their views of the future – melting icecaps, spreading deserts and acidifying oceans – are cautious evaluations that most probably underestimate the likely impact of global warming.
Miami
Miami is built on top of porous limestone and its foundations are now absorbing water from rising seas at an alarming rate. Water now bubbles up through pipes and drains and taints fresh water supplies while seawater regularly flows out of drains into streets, which become flooded. Civil engineers currently estimate that the cost of upgrading the drainage systems in Miami would cost billions of dollars and that, please note, is the price that a single city will have to pay to deal with just one aspect of global warming. Repeat it across the globe and you get a notion of the vast cost we now face for having failed to deal with climate change for the past two decades and for faltering in our commitment to agree to curb emissions of carbon dioxide from our factories, power plants and cars.
The result of this continued inaction has been straightforward: climate change – once a far-off threat – is now upon us and is already bringing alarming change to our planet, as the citizens of Miami are now experiencing, along with those living near spreading deserts in Africa, in the far north where tundras are melting, and in high mountain areas in the Andes and Himalayas whose glaciers are now disappearing. As Leonard Berry, director of the Florida Centre for Environmental Studies at Florida Atlantic University puts it: "Climate change is not a future thing, it's a 'happening now' thing." It is a reality that has already arrived and is destined to have increasingly profound impacts until we wake up to the threat and act coherently.
A woman on a moped navigates a flooded street corner on Miami Beach, an all-too-familiar sign for residents of this iconic peninsula where the ocean seems more likely than ever to swamp Ocean Drive one day.
If there's an image that starkly illustrates the threats of climate change, it's this photograph, which was included in the recent National Climate Assessment released by the White House. It is noteworthy because the flood is from exceptionally high spring tides – not heavy rains. Tidal flooding like that is relatively new. And scary. "People in Miami Beach are living climate change," said David Nolan, a meteorology and physical oceanography professor at the University of Miami. "They're on the frontline."
The people of Miami Beach didn't need the National Climate Assessment to tell them low-lying south Florida is "exceptionally vulnerable to sea level rise". The city is already spending $206m to overhaul its drainage system.
The day after the White House released its climate change report, Miami-Dade County's commission passed a 6 May resolution that calls on planners to account for sea level rise. Local officials across the four counties of south Florida are making similar moves. Almost anyone who lives in south Florida has a nagging fear about climate change. It's both abstract and, at times, very real.
One of the most vulnerable places in South Florida: Key West. Homes were flooded by hurricane Wilma in 2005. It was the first flood of its kind and Wilma was the last of an unprecedented eight hurricanes that damaged parts of Florida in 13 months. Talk of global warming mounted after those back-to-back mean seasons. No hurricanes have directly hit since. Widespread discussion about climate change subsided.
Here's what hasn't gone away: rising home insurance rates, which have increased in part because insurance firms do believe in climate change. And then there's the concern that the one investment that we see as the fulfilment of the American dream – our homes – will be impossible to keep or will be so devalued in 30 years that they won't be worth passing on to our children.
Sea level rise threatens our drinking water supplies, farm fields and the main driver of tourism: beaches. But in the absence of a disaster, it's easy for many of us to forget about the long-term risks until a report such as the National Climate Assessment details them. But while local officials are talking about solutions and planning, politicians farther away from south Florida aren't. The issue has become an ideological fault line on the state and national levels. In the state capital in Tallahassee, Republican governor Rick Scott stalled efforts to grapple with the issue.
In Washington, Republican senator Marco Rubio is emerging as a leading sceptic of man-made climate change. A potential 2016 candidate for president, he said that scientists sometimes get it wrong. "Some of these people were predicting a new ice age in the 1970s. Imagine if we had acted on that."
In the Democratic-leaning Miami area, people are not inclined to side with Rubio. But many hope that he's right. Hope does nothing to stop the seas from rising. "Climate change is real," said Nolan. "It's here." And it's clear to anyone walking around South Beach when there's high spring tide.
Antarctica
There is another, more straightforward reason to repudiate deniers' claims about scientists' "false alarms", however. The impact of climate change is not an issue that is going to be determined in far-off years for the simple reason that it is already happening. This is a point made clear by Nasa glaciologist Eric Rignot who reveals that his observations show that a large part of the West Antarctica ice sheet has now begun to disintegrate and that the entire sheet appears today to be in irreversible retreat.
"One of the feared tipping points of the climate system appears to have been crossed," says Stefan Rahmstorf, an expert on the physics of the ocean at Potsdam University. Certainly the consequences of this massive destabilisation of ice cover at the south pole are going to be considerable, scientists now argue
The last assessment report of the Intergovernmental Panel on Climate Change (IPCC) put a modest figure of one to three feet as the likely rise in sea levels that will be experienced this century. The disintegration of the entire West Antarctic ice shelf extends that forecast drastically. A figure of 10 feet is now a more than likely option over the coming centuries. Vast tracts of heavily populated coastline around the world face inundation. Millions are likely to lose their homes. It may take more than a century for this devastation to occur. Nevertheless, it now looks to be inevitable, says Rignot. Nor will the residents of low-lying regions such as Bangladesh or Florida be surprised at this forecast. They are already experiencing the consequences of rising sea levels triggered by melting icecaps.
Last Monday, we hosted a Nasa conference on the state of the West Antarctic ice sheet, which, it could be said, provoked something of a reaction. "This Is What a Holy Shit Moment for Global Warming Looks Like," ran a headline in Mother Jones magazine.
We announced that we had collected enough observations to conclude that the retreat of ice in the Amundsen sea sector of West Antarctica was unstoppable, with major consequences – it will mean that sea levels will rise one metre worldwide. What's more, its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea level rise of between three and five metres. Such an event will displace millions of people worldwide.
Two centuries – if that is what it takes – may seem like a long time, but there is no red button to stop this process. Reversing the climate system to what it was in the 1970s seems unlikely; we can barely get a grip on emissions that have tripled since the Kyoto protocol, which was designed to hit reduction targets. Slowing down climate warming remains a good idea, however – the Antarctic system will at least take longer to get to this point.
The Amundsen sea sector is almost as big as France. Six glaciers drain it. The two largest ones are Pine Island glacier (30km wide) and Thwaites glacier (100km wide). They stretch over 500km.
Many impressive scientists have gone before us in this territory. The concept of West Antarctic instability goes back to the 1970s following surveys by Charles Bentley in the 1960s that revealed an ice sheet resting on a bed grounded well below sea level and deepening inland. Hans Weertman had shown in 1974 that a marine-based ice sheet resting on a retrograde bed was unstable. Robert Thomas extended his work to pursue the instability hypothesis. Terry Hughes suggested that the Pine Island sector of West Antarctica was its weak underbelly and that its retreat would collapse the West Antarctic ice sheet. Considerable uncertainty remained about the timescale, however, due to a lack of observation of this very remote area.
Things changed with the launch of the ERS-1 satellite which allowed glaciers in this part of antartica to be observed from space. In 1997, I found that the grounding line (where the glacier detaches from its bed and becomes afloat) of Pine Island glacier had retreated five kilometres in the space of four years, between 1992 and 1996. Stan Jacobs and Adrian Jenkins had found a year earlier that the glacier was bathing in unusually warm waters, which suggested the ocean had a major influence on the glacier. Duncan Wingham and others showed that the glacier was thinning. In 2001, I found that Thwaites glacier was retreating too .
At that point, the scientific community took a different look at the region. Work by the British Antarctic Survey, Nasa and Chile led to more detailed observations, a monitoring programme was initiated, instruments were placed on the ice, in the ocean and scientific results started to pile up from a variety of research programmes. From that point, we all sought to find out whether this was really happening. Now, two decades after this process started, we have witnessed glacier grounding lines retreat by kilometres every year, glaciers thinning by metres every year hundreds of kilometres inland, losing billions of tons of water annually, and speeding up several percent every year to the flanks of topographic divides.
Thwaites glacier started to accelerate after 2006 and in 2011 we detected a huge retreat of the glacier grounding lines since 2000. Detailed reconstructions of the glacier bed further confirmed that no mountain or hill in the back of these glaciers could act as a barrier and hold them up; and 40 years of glacier flow evolution showed that the speed-up was a long story.
All these results indicate a progressive collapse of this area. At the current rate, a large fraction of the basin will be gone in 200 years, but recent modelling studies indicate that the retreat rate will increase in the future. How did this happen? A clue is that all the glaciers reacted at the same time, which suggested a common force that can only be the ocean. Ocean heat is pushed by the westerly winds and the westerlies have changed around Antarctica in response to climate warming and the depletion of the ozone. The stronger winds are caused by a world warming faster than a cooling Antarctica. Stronger westerlies push more subsurface warm waters poleward to melt the glaciers, and push surface waters northward.
Nerilie Abram and others have just confirmed that the westerlies are stronger now than at any other time in the past 1,000 years and their strengthening has been particularly prominent since the 1970s as a result of human-induced climate warming. Model predictions also show that the trend will continue in a warming climate.
What this means is that we may be ultimately responsible for triggering the fast retreat of West Antarctica. This part of the continent was likely to retreat anyway, but we probably pushed it there faster. It remains difficult to put a timescale on it, because the computer models are not good enough yet, but it could be within a couple of centuries, as I noted. There is also a bigger picture than West Antarctica. The Amundsen sea sector is not the only vulnerable part of the continent. East Antarctica includes marine-based sectors that hold more ice. One of them, Totten glacier, holds the equivalent of seven metres of global sea level.
Controlling climate warming may ultimately make a difference not only about how fast West Antarctic ice will melt to sea, but also whether other parts of Antarctica will take their turn. Several "candidates" are lined up, and we seem to have figured a way to push them out of equilibrium even before warming of air temperature is strong enough to melt snow and ice at the surface.
Unabated climate warming of several degrees over the next century is likely to speed up the collapse of West Antarctica, but it could also trigger irreversible retreat of marine-based sectors of East Antarctica. Whether we should do something about it is simply a matter of common sense. And the time to act is now; Antarctica is not waiting for us.
Eric Rignot is a glaciologist at NASA's Jet Propulsion Laboratory. He is the lead author of last week's landmark scientific paper on West Antartica
