The paper is only available to people with access to scientific journal articles. Fortunately, Hansen has provided a publicly accessible brief online.

Sadly, though, the brief is very hard to understand. It’s probably only comprehensible to those who have a scientific background and a good working knowledge of recent work in climate science, including Hansen’s own work.

A typical working journalist would tend to either not understand Hansen’s brief at all, or misunderstand the points Hansen is trying to make. Climate science skeptics and deniers could easily use the brief to make claims of their own, undermining Hansen’s work.

Hansen says that there are three main ways to understand climate change trends of today:

1. Observations of current trends based on past and current observations.
2. Comparisons of current and expected conditions to conditions earlier in the Earth’s history.
3. The use of complicated models that incorporate trend data and scientific theory to project climate change in the decades and centuries to come. These models can be used to estimate the impacts of different steps people take in the near future, such as higher or lower CO2 emissions.

A 2C increase in temperature meant big sea level rises

CO2 levels in the atmosphere have been quite steady since the end of the last Ice Age, 10,000 years ago. This is the period in which civilization developed. Our current state of civilization, marked by the Industrial Revolution, the emergence of CO2-spewing factories, then power plants, cars and airplanes, plus paving over lots of land, has seen CO2 levels rapidly  increase from a previous level around 280ppm to around 390ppm today. Emissions are steadily increasing, currently adding about 2ppm each year to the level of CO2 in the atmosphere.

At this rate – which shows no sign of slowing – we will double the overall CO2 in the atmosphere to double the pre-industrial level, about 560ppm, in this century. (If emissions keep accelerating, doubling could happen closer to mid-century.) Hansen states that this would result in an increase of about 3C – almost 5F – in average global temperatures. He further states that a smaller rise of 2C – about 3.2F – has been seen as an acceptable increase.

His paper holds that an 2C increase is “a prescription for disaster”. In earlier times, an increase of just 1-2C saw sea levels rise to about 15-25m (50-80 feet) higher than today. So, to prevent such a huge rise in sea levels, it’s necessary to hold emissions much lower; in other papers and statements, Hansen has stated that a drop in atmospheric CO2, to 35oppm, is needed to keep ice sheets from melting much further and causing large increases in sea level.

Hansen also points out a difference between fast and slow feedbacks in the climate system. Fast feedbacks are quick changes such as changes in clouds, which increase the effect of CO2 changes by trapping heat. Slow feedbacks are slower changes such as melting sea ice, which replace surface areas of white ice – which reflects sunlight away from Earth before much warming happens – with dark ocean water, which trap nearly all of the sunlight, absorbing its hear. So, as CO2 levels increase, some change will happen in a few years, but more change will come over the following decades.

Hansen doesn’t mention the way in which these feedbacks interact, nor can he say how fast all this might occur.

Unfortunately, even the points Hansen does make are stated in a confusing way. Also, much of the overall point Hansen is making relates to other papers that he’s co-authored in the past, as well as other work that he’s cited approvingly. So Hansen, in the brief, is implying more than he’s actually saying. And, sadly, the implications as well as the conclusions are all knotted together in a way that’s hard to understand.

Hansen fails to state his core conclusions clearly and leaves what he does say open to misinterpretation, whether accidental or deliberate. Unfortunately, his valuable work is not presented in a way that helps the cause of understanding, let alone acting on, current levels of climate change.