Dangerous climate change

It must be about time I wrote something about global warming again. I want to write about a new paper by James Hansen – the world’s best known climate scientist and former head of NASA’s Goddard Institute for Space Studies. It’s called Assessing “Dangerous climate change”: required reductions of carbon emissions to protect young people, future generations and nature. [Note that he is not the only author of this paper but he’s the first listed so for simplicity I’m going to refer to James Hansen only.]

A little while ago I wrote a post about climate change targets in which I suggested that the internationally agreed temperature limit of 2°C was a somewhat arbitrary figure to choose as a target for the maximum temperature we are prepared to tolerate. Hansen’s latest paper makes the case for a temperature limit of just 1°C because he says 2°C would activate slow feedbacks in the climate system and result in dangerous warming of 3-4°C.

So what are the dangerous impacts he’s talking about? Some of these are food shortages, extreme weather events, an increase in extreme summer heat, world sea level rise, ocean acidification and scarcity of water supplies. Hansen describes the climate impacts of a 2°C warmer world as “disastrous”.

I know that for many people 2°C doesn’t sound like very much. After all, our local temperatures vary by much more than 2°C on a day-to-day basis. But a 5°C change in the other direction was enough to “bury a large part of North America” under a thick blanket of ice some 20,000 years ago. During the Eemian period, the interglacial which began 130,000 years ago, the Earth was ~2°C warmer than today and with a sea level peak of 9m above present. That’s a death sentence for many of today’s cities.

Once the temperature has increased we’re stuck with it for a long time – centuries – because the oceans suck up so much heat and transfer it to their depths. The Antarctic and Greenland ice sheets took thousands of years to grow and once they melt, cannot be refrozen so easily. The loss of these ice sheets would raise the sea level by many metres wiping out coastal cities around the world. Paleoclimate data tells us that sea level has risen as fast as one metre per century in the past and we are currently increasing greenhouse gas emissions at a rate not seen before.

But it doesn’t have to be all doom and gloom. If we stopped emissions in 2015, CO2 would decline to 350ppm (we are currently at 395ppm) by the end of the century due to natural sinks – ocean, land and biosphere – that suck it up. If we wait 20 years, then we won’t see a return to 350ppm until 2300. If we wait 40 years, it will take until after 3000 before we see a return to 350ppm. The longer we delay action, the harder the problem is to solve.

Had we started tackling this problem in 2005, an emissions reduction of just 3.5% per year would have led us to 350ppm by 2100. But we didn’t. Now we would need to reduce emissions by 6% per year. If we wait until 2020, we will need to reduce emissions by 15% per year.

By studying how Earth’s climate has changed in the past, Hansen gives his best estimate for climate sensitivity – the amount of warming we can expect per doubling of CO2 – which is close to 3°C. We are expected to reach this point of doubling by mid-century. This doesn’t mean the temperature will have increased 3°C by mid-century because there’s an inertia in the climate system and it will take a little while for it to catch up. But this is the pathway we will have carved for ourselves and our children. It’s worth mentioning that this calculation for climate sensitivity is made with paleoclimate data and is independent of climate models.

He discusses aspects of policy and supports a carbon tax, improvements in energy efficiency and expansion of renewable energies and nuclear power. He also expects reforestation and improved agricultural and forestry practices to absorb some CO2. If the two biggest emitters – China, USA – implement a carbon tax, others will follow suit because those two could impose a border duty on products from countries without a carbon tax.

He also discusses removing CO2 from the atmosphere with geoengineering to highlight the cost and difficulty of doing this. If we could turn 1 GtC (gigatonne) of CO2 into a brick (and note that we have emitted 531 GtC so far and counting) it would produce the volume equivalent of about 3000 Empire State buildings. It is possible, according to Hansen, that with a carbon tax, or some other financial incentive, we may develop the technology to do this efficiently and cost-effectively but the technology is not available today.

Hansen pitches the problem as a moral one of intergenerational ethics and I agree with him on this. The idea here is that future generations will inherit a climate situation that is spiralling out of control but for which they are not to blame. Yet they have a fundamental right to inhabit a liveable planet.

Although the paper is about impending disaster, it is only a disaster if we do nothing. Hansen ends with the conviction that this is not an insurmountable problem. It can be solved if we take appropriate steps today and he makes clear in the paper what these steps are: a rising fee on fossil fuel emissions, reforestation and improvements in agricultural and forestry practices and widespread adoption of renewable and carbon-free energy.

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