SPIEGEL: Dr. Hansen, temperatures in Central Park reached 22 degrees Celsius (71.6 degrees Fahrenheit) in January, the Alps lacked snow throughout the season -- is winter being erased by global warming?
James Hansen: Weather and climate are two different things, which often confuses people. The average temperature is now 0.8 degrees Celsius higher than in the last century, with three-quarters of the increase happening in the last 30 years. But compared to the usual weather fluctuations, thats quite small. You can have a ten-degree variation from day to day in the weather. You can have anomalies of an average of several degrees over one month, and you still dont have to worry. However, this does not indicate we can stop thinking about this issue: We've just experienced the warmest January in 127 years of record-keeping.
SPIEGEL: Even so, a global warming of 0.8 degrees Celsius over 100 years doesnt sound very dramatic.
Hansen: There's another half degree Celsius in the pipeline due to gases already in the atmosphere, and there's at least one more half degree to come due to power plants which we're not going to stop immediately. Even if we decide now we've got to slow down as fast as is practical, there's still going to be enough emissions to take us to the warmest level that the planet has seen in a million years. That's enough to take us close to and possibly beyond what I would say is a dangerous level.
SPIEGEL: How can you be so sure of that?
Hansen: We know a lot about the history of the Earth. If we want to keep the planet looking close to what it looks like now, then we had better not accept an increase by more than one degree Celsius. Because if temperature goes up another two or three degrees Celsius, it will be the temperature of the middle Pliocene about 3 million years ago. That was a very different planet. There was no sea ice in the Arctic in the warm seasons, and the sea level was about 25 meters higher. We will be headed towards this situation if we continue with business-as-usual.
SPIEGEL: Who or what is responsible for the warming of our planet?
Hansen: I would say it's approximately 102 percent man-made. We know very well that the climate would have been heading towards a colder climate if it weren't for human-made emissions. We've been in an interglacial period now for almost 12,000 years, and we know that this period peaked 6,000 or 8,000 years ago. We were headed slowly toward cooler climates until humans came along. And now humans have sent us off in a completely different direction.
SPIEGEL: Just recently the United Nations Panel on Climate Change declared that it is now 90 percent certain that global warming is real. Doesnt this come a bit late?
Hansen: You know, we said that a long time ago, but that's okay. This panel involves more than one hundred nations including Saudi Arabia and others. Those countries have to be dragged kicking and screaming to this sort of conclusion. The panel, by the way, is very reluctant to say anything about sea level change, although the evidence that has accumulated in the last two or three years is impressive.
SPIEGEL: What does it show?
Hansen: We've gotten fantastic measurements from a gravity satellite. They explain exactly how Greenland and West Antarctica are changing in mass and how much mass they're losing to the ocean. We have other observations of ice quakes on Greenland and ice streams speeding up, and we see processes occurring which make me very concerned about the stability of ice sheets.
SPIEGEL: What exactly do you fear will happen?
Hansen: The disintegration of ice sheets will be a non-linear process, and that means it can change very rapidly. You can have relatively slow changes for a while, but once you reach a certain instability, you get sudden collapse and a very large change. We know pretty well from the history of the earth that when ice sheets have disintegrated in the past, they have disintegrated very rapidly. During the last melting period, the sea level went up 20 meters in 400 years, which is one meter every 20 years.
SPIEGEL: What makes you so sure about such a dramatic melting this time? Other scientists claim this process may take thousands of years.
Hansen: I think thats a very dangerous assumption. I would be very surprised if we didn't get a big change this century. We need to commission a study of this problem and bring in the best scientists so that they can look at it. We need to take action soon.
SPIEGEL: Youve made the point that we have only 10 more years to prevent the worst consequences of global warming. Why?
Hansen: Lets contrast two different scenarios. The first one I call business-as-usual which is the typical UN scenario. It shows a continued increase in the annual emissions of CO2 of about 1 or 2 percent per year. That's what leads you to at least 2 to 3 degrees Celsius global warming in this century. An alternative scenario is designed to keep global warming at about one additional degree or less. It requires that CO2 emissions actually decrease on a global average by at least a few tens of percent by mid-century. By the end of the century, you have to stabilize things, which means you would need a 60 to 80 percent reduction in emissions.
SPIEGEL: What happens precisely if your business-as-usual scenario applies?
Hansen: This will put us 40 percent above our 2000 emissions ten years from now. Then it will become very difficult to halt global warming and get back to the alternative scenario.
SPIEGEL: What does this mean? When will New York be flooded?
Hansen: Thats hard to answer because it is a non-linear process. But this much is clear: the sea level, which up until a century ago was rather stable, increased by about 15 centimeters in the last century and is now increasing at a rate of 35 centimeters per century, 3.5 centimeters per decade. So the rate has gone up, and if it ratchets up a few more times, pretty soon you're talking about really significant change.
SPIEGEL: What other effects do you expect besides this?
Hansen: Im really concerned about the extermination of species. If climatic zones move, animals and plants need to migrate. Studies have found that 1,700 species have already moved poleward at a rate of six kilometers per decade in recent decades. But climate zones are moving poleward at a faster rate, about 50 km per decade, and it will become 100 km per decade with business as usual. Combine that with the fact that so many species have been confined to certain areas due to humans having taken over so much of the planet, and youll see it may be very difficult for them to migrate. So it's likely that a large fraction of the species could go extinct.
SPIEGEL: Which nation on earth is most responsible for global warming?
Hansen: Some US politicians are making the argument that China is soon going to be the biggest emitter of greenhouse gases, and that's true. Within a few years, they will pass the US. But climate change depends upon the cumulative emissions over time because much of the CO2 we emitted in 1850 is still here and its still damaging. So its not only about the present emissions. Therefore, the US is responsible for more than three times the amount of emissions than any other country, with China and Russia being next, and Germany and Great Britain after that.
SPIEGEL: George W. Bush refused to sign the Kyoto protocoll right at the beginning of his presidency, and he declared he doesnt want to do anything that might hurt the US economy.
Hansen: Who is going to bear the moral burden? The politicians who deny there is a problem today will no longer be in office once the effects of global warming are felt. The frustrating part is that the solutions actually make sense for other reasons.
SPIEGEL: Because changes in US energy use would reduce Americas dependence on foreign oil?
Hansen: Exactly. Our waste of energy has resulted in all kinds of problems, including the current ones in the Middle East.
SPIEGEL: What needs to be done?
Hansen: We are still facing a solvable problem. Oil and gas are running out anyway and we should use that resource very conservatively. It's amazing how much energy is compacted into these fossil fuels. The architects and building engineers tell us they can now make buildings which use only half as much fossil fuel as current buildings. We have not improved our vehicle efficiency since about 1980. So there is a lot of potential for energy saving. Yet we are wasting these resources. Furthermore, instead of shutting down existing coal plants across the country, American energy corporations are planning to construct 100 new ones.
SPIEGEL: What can people do on a personal level to help slow climate change?
Hansen: If an individual goes out and reduces his emissions, it's a nice example and it shows that it can be done. But even if a lot of people comply, what is the result? It reduces the price of fuel, and then it'll be so cheap, somebody else will burn it.
SPIEGEL: What do you propose instead?
Hansen: The most important thing that people can do is influence the government. The most critical policy element has to be a slowly growing price on carbon emissions. It has to be fast enough to have an impact and affect industries and their investments and innovations. But it has to be slow enough so there is time for these new technologies to develop, so consumers can choose and buy new, more efficient technologies. We should have started on that a long time ago.
The interview was conducted by Frank Hornig