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At the suggestion of Slovenian colleagues, James Hansen has written to the President and members of the National Assembly of the Republic of Slovenia urging them to deny a state guarantee for a proposed European Investment Bank loan to fund a new lignite-fired power plant in their country.

He points out that they are considering a decision which will have significant effects, some irreversible, upon the world that today’s young people and future generations inherit. Such a strong statement, he says, however unlikely it may seem at first glance, is a clear conclusion of the most advanced climate science. That science he proceeds to summarise in his letter, and to describe in more detail in an attached paper The Case for Avoiding Dangerous Climate Change to Protect Young People and Nature.

The paper is well worth attention. No fewer than 17 co-authors, from a variety of universities and institutes around the world, participated in its preparation. The result is a valuable summation of the science and the implications for humanity in moving to carbon-free energies and energy efficiency. I don’t know whether many politicians actually sit down and read a coherent account of where climate science is at, but the paper serves extremely well for anyone, politician or not, who wants such an account.

’Humanity is now the dominant force driving changes of Earth’s atmospheric composition and thus future climate.’ Hansen’s blunt opening statement is explained in detail as the paper proceeds. I’ll touch only lightly on its contents, which are familiar enough to anyone who follows the issue. It’s worth noting the increasing urgency which the paper points to as the impacts of global warming become apparent. What was once thought to be a tolerable level of a few degrees of warming can now be seen as dangerous. Although global warming is currently less than 1ºC, significant impacts are already apparent. There has been a much larger than expected decrease in summer Arctic sea ice; the Greenland and Antarctic ice sheets are shedding ice at several hundred cubic kilometres per year and accelerating; mountain glaciers around the world are receding rapidly; the hot dry sub-tropical climate belts have expanded; the abundance of reef-building corals is decreasing; more than half of all wild species are experiencing habitat and seasonal timing changes; mega-heatwaves have become more widespread.

There is a consequent need for a reassessment of what constitutes a dangerous level of warming and the paper points to Earth’s paleoclimate history as a guide. Powerful feedbacks of loss of albedo and rising CO2 emissions greatly amplified the effects of what would otherwise have been small changes in past climates; that is a warning for us of what the extremely rapid rise in emissions today may result in. Past records of sea level rise are particularly ominous in this respect.

The paper explains the conclusion that an atmospheric CO2 concentration of 350 ppm is the target which could stabilise climate near current temperatures. It’s an important conclusion, but also a troubling one in that we are already well past that mark and climbing. However the paper considers it is not impossible to return to 350 ppm this century through reforestation and increasing soil carbon. But emissions must start to be scaled back urgently as well.

Hansen et al also warn of the possible development of slow feedbacks, such as ice sheet disintegration, species extinction or the release of methane hydrates, if warming is allowed to continue. These slow feedbacks can become tipping points where further and possibly rapid changes become inevitable because they develop their own momentum and the dynamics of the process takes over.

The paper runs through the likely impacts of global warming — sea level rise, continuing into successive centuries possibly to very high levels; shifting climate zones, already apparent in isotherms moving poleward at a typical rate of the order of 100 km/decade in the past three decades; threats to species survival; loss of coral reefs; climate extremes; a variety of threats to human health.

The only effective response to the dangers posed by warming is for the world to move expeditiously to carbon-free energies and energy efficiency, leaving most of the remaining fossil fuels in the ground. This transition, the paper considers, will not occur until a price is put on carbon. Fossil fuels are cheap only because they are subsidised and do not pay their costs to society.

One of the world’s leading climate scientists, supported by a distinguished group of colleagues from a number of disciplines, takes the trouble to write to the members of the Slovenian National Assembly over one coal-fired power plant, and to include a carefully prepared paper tailored to their understanding.  One hopes the Assembly members will recognise that the attention they have received is a measure of the concern that the authors feel that the world is heading for major disaster.  One also hopes they will read the paper closely. I did, and I find it difficult to see how anyone could doubt the overwhelming weight of scientific evidence it canvasses that we are at a time when it is imperative that we begin a substantial and steadily continuing reduction in our emission of greenhouse gases and that we acknowledge that most remaining fossil fuel reserves must stay in the ground.

Some social scientists point out that the scientific evidence alone is not what convinces people. I have some difficulty appreciating this because I found some years ago that the science was quite enough to convince me. They in return would no doubt say that I was socially pre-conditioned to conviction. But whatever the merits of such arguments it surely remains important that the stark reality of the scientific picture be continually brought to public attention and that we are made to understand that the scientists themselves are deeply alarmed by what they discover. That’s the touchstone against which all our responses should be measured, whatever social factors may also come into play. Hansen and his colleagues perform a public service of high importance.