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Sea level rise: drowning in numbers
OPINION
Drowning in numbers We know that sea level will rise, but how far and how fast? The latest attempt to put figures on it is dangerously misleading, says Michael Le Page IMAGINE your job is to protect London from surging seas. In one way it is easy: unlike most coastal cities, London has a formidable flood defence system in the form of the Thames Barrier, capable of protecting it from all but the highest storm surges. But as the seas rise, the risk of the barrier being breached will increase steadily. With a 1-metre rise in local sea level, London will get flooded every 10 years. So when do you start building new flood defences, and how high do you make them? The stakes are enormous. Building new defences will cost tens of billions and involve decades of planning and controversy before construction even begins. Get it wrong, and storm surges could kill thousands and displace millions. So all around the world, planners are clamouring to know how fast the seas will rise as the planet warms. Until recently, scientists could not give them any reliable numbers. There were no computer models capable of simulating the melting of the world’s ice sheets and glaciers. The 2007 report of the Intergovernmental Panel on Climate Change (IPCC) handled this uncertainty really badly. It acknowledged that we don’t know how fast all the ice will melt, but then gave some numbers anyway – between 18 and 59 centimetres of sea level rise by 2100 – based on highly dubious assumptions such as glaciers continuing to flow at the same rate and the Antarctic ice sheet growing larger. The numbers also assumed a maximum 26 | NewScientist | 25 May 2013
warming of 5.4 °C, even though the report’s highest projection was 6.4 °C. Unsurprisingly, many people wrongly took 59 cm of sea level rise to be the worst case. Now we have some more numbers. A European-funded project called ice2sea has developed computer models of glaciers and ice sheets. Earlier this month it announced that melting ice would contribute between 4 and 37 cm to global sea level by 2100. Adding this to the other causes of sea level rise – the main one being the expansion of the oceans as they warm – gives figures of between 16 and 69 cm by 2100. Some media reports focused
on the fact that this is less than some other recent estimates of at least a metre. “Seas will rise no more than 69 centimetres by 2100,” proclaimed this magazine. Others focused on the fact that even this relatively small rise could have devastating consequences. “Floods could overwhelm Thames Barrier by end of century,” declared The Guardian in London. How much trust can we put in these numbers, though? The whole point of the ice2sea programme was to “reduce the uncertainty”,
“The projections for how much sea level will rise imply a level of certainty that simply doesn’t exist”
but its numbers come with some rather large caveats. For starters, the modellers didn’t have the computing power to look at a range of scenarios for how much carbon dioxide we will pump into the atmosphere. Instead, they looked at just one – a “mid-range” scenario predicted by the 2007 report to lead to warming of around 3 °C. Yet actual emissions today are much closer to the worst-case scenario, which some recent studies predict could lead to warming of 6 °C or more. And far from falling, annual global emissions are rising ever faster. With hundreds more coal-fired power stations being built and new sources of fossil fuels like tar sands being exploited, there is good reason to think emissions will continue to soar for many decades to come. What’s more, to account for the fact that warming will not be uniform across the globe, the modellers had to produce regional projections of warming, snowfall and so on to feed into the ice models. But regional projections are highly unreliable, with different models often producing wildly varying results. The prime example is the Arctic, where the sea ice is disappearing much faster than anyone expected. To understand why regional climate predictions are so much less reliable than global ones, think of the heat entering the atmosphere and oceans as water pouring into a bath. Predicting the average level of the bath is much easier that predicting the height of the waves sloshing around.
Comment on these stories at newscientist.com/opinion
Michael Le Page is a features editor at New Scientist
One minute with...
Ralph Keeling For the atmospheric scientist, monitoring carbon dioxide is a family tradition. He says we've reached a worrying milestone The Mauna Loa Observatory in Hawaii has reportedly recorded a carbon dioxide concentration there of 400 parts per million for the first time. How significant is that? It’s a psychological milestone. Every year in the last few decades, CO2 concentrations have been going up by about 2 ppm per year. Those changes go unnoticed but people pay attention to round numbers. It gives you a bit of perspective on how far we’ve come – a bit like turning 40, or 50. So how far have we come? Before the industrial revolution, we started at about 280 ppm. 100 years ago, levels had risen to around 300, and they crossed 350 in the late 1980s. We think the last time concentrations were as high as 400 ppm was between 3 and 5 million years ago, when the world was much warmer. What did Earth look like 3-5 million years ago? It had much higher sea levels, forests extended all the way to the Arctic Ocean, and there was almost certainly a lot less sea ice. Today, sea ice is melting rapidly, and in the last decades we have seen the tree line moving north into the Arctic tundra. Are we in a climate danger zone? In my view, yes. At 400 ppm, we’ve perturbed Earth enough already that things could unfold that will be catastrophic. We passed 400 ppm for the first time last year, above the Arctic. What is special about the Mauna Loa record? It’s the one record that has high resolution going back to the late 1950s – when my father set it up. Why did he start tracking CO₂ at Mauna Loa? In the early 1950s, he was at the California Institute of Technology studying carbon in rivers. As part of that, he developed a way to measure CO₂ in the air. He discovered that if you measured concentrations in a sufficiently remote place, you almost always got the same number. That was unexpected. Previous work suggested CO₂ levels were more variable, making measurement very
Profile Ralph Keeling directs the CO₂ programme at the Scripps Institute of Oceanography in La Jolla, California. He is pictured here with his pioneering father, the late Charles David Keeling
difficult. The realisation that there was a stable background level meant the challenge of measuring the increase might not be so great. You simply had to go to a place far enough from contamination and track it over time. The Mauna Loa measurements came later, beginning in 1958. When did he first see a steady rise in CO₂ – what is now known as the Keeling curve? The early days at Mauna Loa were fraught. Power outages meant the measuring instrument had to be shut down for weeks. It would come back on reading a different level. He thought there should be a stable background, but concentrations were fluctuating. It was only when he’d gathered a year of data that he realised there was a seasonal cycle. So levels may drop below 400 ppm again? Crossing from below to above 400 will play out over years, partly because there is a natural up and down with the seasons. But this time next year it will be higher still. In a couple of years we’ll never get below 400 again. Interview by Catherine Brahic
25 May 2013 | NewScientist | 27
Scripps Institute
So the climate information being fed into these latest ice models could be way off the mark. And even if it isn’t, how do we know the models are right? Well, say the researchers, they can reproduce some of the observed responses to the actual 0.5 °C warming of the past few decades, such as the retreat of glaciers. But that doesn’t prove they can predict the response to future warming of 3 or 6 °C. There are similar issues with global climate models. This kind of research is vital. But when such a limited study is presented as the “best estimate” available, the danger is that it will be misinterpreted in the same way as the 2007 IPCC report. Its numbers do not encompass the worst-case scenario – far from it. They don’t even represent the most likely scenario. The narrow range implies a degree of certainty that simply doesn’t exist. Nobody should be basing life-and-death decisions such as how to protect Londoners on these numbers. The ice2sea organisers must have been aware of this because they also asked a bunch of experts how bad they thought it could get. This exercise produced yet another set of numbers – there is less than a 1 in 20 chance that sea level rise will exceed 84 cm by 2100. But isn’t the whole point of modelling this stuff to reduce our reliance on guesstimates? The big picture is that there is no doubt that the planet will get hotter and that sea level will eventually rise many metres. We know this because the last time atmospheric CO2 levels were higher than 400 parts per million, sea level was between 5 and 40 metres higher. Even if emissions stopped tomorrow, there would still be huge sea level rises. The only question is how fast it will happen. The frightening truth is that we still don’t know. ■
Drowning in numbers We know that sea level will rise, but how far and how fast? The latest attempt to put figures on it is dangerously misleading, says Michael Le Page IMAGINE your job is to protect London from surging seas. In one way it is easy: unlike most coastal cities, London has a formidable flood defence system in the form of the Thames Barrier, capable of protecting it from all but the highest storm surges. But as the seas rise, the risk of the barrier being breached will increase steadily. With a 1-metre rise in local sea level, London will get flooded every 10 years. So when do you start building new flood defences, and how high do you make them? The stakes are enormous. Building new defences will cost tens of billions and involve decades of planning and controversy before construction even begins. Get it wrong, and storm surges could kill thousands and displace millions. So all around the world, planners are clamouring to know how fast the seas will rise as the planet warms. Until recently, scientists could not give them any reliable numbers. There were no computer models capable of simulating the melting of the world’s ice sheets and glaciers. The 2007 report of the Intergovernmental Panel on Climate Change (IPCC) handled this uncertainty really badly. It acknowledged that we don’t know how fast all the ice will melt, but then gave some numbers anyway – between 18 and 59 centimetres of sea level rise by 2100 – based on highly dubious assumptions such as glaciers continuing to flow at the same rate and the Antarctic ice sheet growing larger. The numbers also assumed a maximum 26 | NewScientist | 25 May 2013
warming of 5.4 °C, even though the report’s highest projection was 6.4 °C. Unsurprisingly, many people wrongly took 59 cm of sea level rise to be the worst case. Now we have some more numbers. A European-funded project called ice2sea has developed computer models of glaciers and ice sheets. Earlier this month it announced that melting ice would contribute between 4 and 37 cm to global sea level by 2100. Adding this to the other causes of sea level rise – the main one being the expansion of the oceans as they warm – gives figures of between 16 and 69 cm by 2100. Some media reports focused
on the fact that this is less than some other recent estimates of at least a metre. “Seas will rise no more than 69 centimetres by 2100,” proclaimed this magazine. Others focused on the fact that even this relatively small rise could have devastating consequences. “Floods could overwhelm Thames Barrier by end of century,” declared The Guardian in London. How much trust can we put in these numbers, though? The whole point of the ice2sea programme was to “reduce the uncertainty”,
“The projections for how much sea level will rise imply a level of certainty that simply doesn’t exist”
but its numbers come with some rather large caveats. For starters, the modellers didn’t have the computing power to look at a range of scenarios for how much carbon dioxide we will pump into the atmosphere. Instead, they looked at just one – a “mid-range” scenario predicted by the 2007 report to lead to warming of around 3 °C. Yet actual emissions today are much closer to the worst-case scenario, which some recent studies predict could lead to warming of 6 °C or more. And far from falling, annual global emissions are rising ever faster. With hundreds more coal-fired power stations being built and new sources of fossil fuels like tar sands being exploited, there is good reason to think emissions will continue to soar for many decades to come. What’s more, to account for the fact that warming will not be uniform across the globe, the modellers had to produce regional projections of warming, snowfall and so on to feed into the ice models. But regional projections are highly unreliable, with different models often producing wildly varying results. The prime example is the Arctic, where the sea ice is disappearing much faster than anyone expected. To understand why regional climate predictions are so much less reliable than global ones, think of the heat entering the atmosphere and oceans as water pouring into a bath. Predicting the average level of the bath is much easier that predicting the height of the waves sloshing around.
Comment on these stories at newscientist.com/opinion
Michael Le Page is a features editor at New Scientist
One minute with...
Ralph Keeling For the atmospheric scientist, monitoring carbon dioxide is a family tradition. He says we've reached a worrying milestone The Mauna Loa Observatory in Hawaii has reportedly recorded a carbon dioxide concentration there of 400 parts per million for the first time. How significant is that? It’s a psychological milestone. Every year in the last few decades, CO2 concentrations have been going up by about 2 ppm per year. Those changes go unnoticed but people pay attention to round numbers. It gives you a bit of perspective on how far we’ve come – a bit like turning 40, or 50. So how far have we come? Before the industrial revolution, we started at about 280 ppm. 100 years ago, levels had risen to around 300, and they crossed 350 in the late 1980s. We think the last time concentrations were as high as 400 ppm was between 3 and 5 million years ago, when the world was much warmer. What did Earth look like 3-5 million years ago? It had much higher sea levels, forests extended all the way to the Arctic Ocean, and there was almost certainly a lot less sea ice. Today, sea ice is melting rapidly, and in the last decades we have seen the tree line moving north into the Arctic tundra. Are we in a climate danger zone? In my view, yes. At 400 ppm, we’ve perturbed Earth enough already that things could unfold that will be catastrophic. We passed 400 ppm for the first time last year, above the Arctic. What is special about the Mauna Loa record? It’s the one record that has high resolution going back to the late 1950s – when my father set it up. Why did he start tracking CO₂ at Mauna Loa? In the early 1950s, he was at the California Institute of Technology studying carbon in rivers. As part of that, he developed a way to measure CO₂ in the air. He discovered that if you measured concentrations in a sufficiently remote place, you almost always got the same number. That was unexpected. Previous work suggested CO₂ levels were more variable, making measurement very
Profile Ralph Keeling directs the CO₂ programme at the Scripps Institute of Oceanography in La Jolla, California. He is pictured here with his pioneering father, the late Charles David Keeling
difficult. The realisation that there was a stable background level meant the challenge of measuring the increase might not be so great. You simply had to go to a place far enough from contamination and track it over time. The Mauna Loa measurements came later, beginning in 1958. When did he first see a steady rise in CO₂ – what is now known as the Keeling curve? The early days at Mauna Loa were fraught. Power outages meant the measuring instrument had to be shut down for weeks. It would come back on reading a different level. He thought there should be a stable background, but concentrations were fluctuating. It was only when he’d gathered a year of data that he realised there was a seasonal cycle. So levels may drop below 400 ppm again? Crossing from below to above 400 will play out over years, partly because there is a natural up and down with the seasons. But this time next year it will be higher still. In a couple of years we’ll never get below 400 again. Interview by Catherine Brahic
25 May 2013 | NewScientist | 27
Scripps Institute
So the climate information being fed into these latest ice models could be way off the mark. And even if it isn’t, how do we know the models are right? Well, say the researchers, they can reproduce some of the observed responses to the actual 0.5 °C warming of the past few decades, such as the retreat of glaciers. But that doesn’t prove they can predict the response to future warming of 3 or 6 °C. There are similar issues with global climate models. This kind of research is vital. But when such a limited study is presented as the “best estimate” available, the danger is that it will be misinterpreted in the same way as the 2007 IPCC report. Its numbers do not encompass the worst-case scenario – far from it. They don’t even represent the most likely scenario. The narrow range implies a degree of certainty that simply doesn’t exist. Nobody should be basing life-and-death decisions such as how to protect Londoners on these numbers. The ice2sea organisers must have been aware of this because they also asked a bunch of experts how bad they thought it could get. This exercise produced yet another set of numbers – there is less than a 1 in 20 chance that sea level rise will exceed 84 cm by 2100. But isn’t the whole point of modelling this stuff to reduce our reliance on guesstimates? The big picture is that there is no doubt that the planet will get hotter and that sea level will eventually rise many metres. We know this because the last time atmospheric CO2 levels were higher than 400 parts per million, sea level was between 5 and 40 metres higher. Even if emissions stopped tomorrow, there would still be huge sea level rises. The only question is how fast it will happen. The frightening truth is that we still don’t know. ■