Hansen: “we are on the verge of setting off irreversible changes.”
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
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
Hansen's 2006 prediction was largely based on the need to avoid
a tipping point where loss of Arctic sea ice becomes
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
study published in Science last week (and summarised by the
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
"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
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
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.