Hansen: “we are on the verge of setting off irreversible changes.”

  • 18 Jan 2011, 15:11
  • Robin

The beginning of 2011 marks five years since James Hansen announced to the world that we have " at most ten years" to tackle rising growth rates of carbon dioxide. Later in 2006 he wrote that we have

"not ten years to decide upon action, but ten years to alter fundamentally the trajectory of global greenhouse emissions."

Five years later, there are few signs that the world is prepared to respond. Global emissions reached an all-time high in 2008, falling only marginally in 2009 as a result of the economic crisis.

So how optimistic is Hansen now? Contacted by Carbon Brief, Hansen sticks by his statement, labelling his choice of decade as "prescient". His more recent research shows that

"…if emissions continue going up another 10 years, from now, there is no way to avoid calamities, except perhaps via extreme geo-engineering actions which [are] implausible and highly undesirable."

Hansen's 2006 prediction was largely based on the need to avoid a tipping point where loss of Arctic sea ice becomes unstoppable.

His argument is that the most effective way to predict the future is to look to past periods in the earth's history. The last time the earth was five degrees warmer (fifty million years ago), global sea level was eighty feet higher; when global temperatures were two degrees higher, sea level was up to sixteen feet higher. That's not to say that these are inevitable or immediate consequences of shifting temperatures, but it does show us how sensitive our planet has been to temperature shifts in the past.

Dr Hansen, a physicist, is the head of the NASA Goddard Institute for Space Studies in New York City. Famous for his testimony to Congress in 1998, he is also known for making strong statements about the future impacts of climate change, going so far as to being arrested on protests against 'mountaintop removal' opencast coal mining in the US.

Not all scientists agree with Hansen that we are on the verge of reaching "tipping points" which will trigger irreversible change. Similar 'backcasting' of the climate, however, is also highlighted by a study published in Science last week (and summarised by the blog Climate Progress), which looks back into geological history for clues about how the climate behaves.

The study demonstrates that if carbon dioxide levels keep rising, they could reach 1,000ppmv by the end of the century (around four times the level prior to the industrial revolution). At times in geological history, levels of carbon dioxide this high have been associated with temperatures 15 degrees C higher than at present, as feedback processes are triggered by rising temperatures, leading to the release of even more carbon dioxide into the atmosphere. At these temperatures, the authors summarise

"human civilization will face another world, one that the human species has never experienced in its history"

The idea of "tipping points" is compelling but surrounded by scientific uncertainties. As Richard Black highlights in a recent blog, however, these kind of analyses do help frame our understanding of the risks the world faces.

When reputable evidence indicates that this level of change is possible, should that have an impact on the world's current response to rapidly rising emissions?

Hansen says his most recent research shows:

"that prior warmer interglacial periods were only several tenths of a degree warmer than the Holocene. The relatively stable climate of the Holocene, with its unusually stable sea level, was at a brink such that several tenths of a degree warming would push polar regions into dramatic climate change with global repercussions. We are on the verge of setting off irreversible changes."

The debates will continue in climate science as more work is done and more evidence unearthed. Even without this evidence, however, the IPCC has already called for global emissions to reverse their trajectory from growth to rapid decrease by 2015. Whatever their point of view, few scientists would disagree that the risks we face if we do not act quickly are very real.

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