The mood in Paris this week as two thousand climate scientists gathered ahead of the United Nations climate summit later this year has been cautiously optimistic. While the window of opportunity to avoid the Earth warming by two degrees Celsius is closing fast, recent political momentum means it is still within reach.

The risk of inaction is profound: we risk destabilizing Earth’s climate and exposing hundreds of millions of people to rising sea levels, shifts in extreme weather, and other impacts.

The Earth’s climate has remained remarkably stable for more than 10,000 years, allowing human civilizations to flourish. Yet we now face the prospect of global temperatures by the end of this century which have not existed on Earth for millions of years, well outside the experience of modern humans. This global warming would threaten and disrupt the lives and livelihoods of hundreds of millions, if not billions, of people across the world.

The scientists in Paris, like the Earth Statement by senior researchers earlier this year, have shown compellingly how these risks are mounting across the world. It is critically important that governments and businesses listen and recognize what is at stake.

In 2010, governments agreed to limit the rise in global average temperature to no more than two Celsius degrees above its pre-industrial level. The Intergovernmental Panel on Climate Change recently estimated that to have a 66% probability of remaining below two degrees, we will have to limit emissions of carbon dioxide from fossil fuels to one trillion tonnes. At current rates of emissions, we will burn through this budget by 2040.

But even if we succeed in limiting future emissions to one trillion tonnes, global warming of 2 Celsius degrees will not be completely safe. For instance, scientists published research today showing evidence that present temperature targets may commit Earth to at least six metres sea level rise.

Even if we do manage to keep within the two-degree guardrail, it would be in terms of average global temperature. Some places would become significantly warmer, with potentially catastrophic effects, and we would face risks of major health and food security impacts contributing to political, social and economic instabilities.

In addition, a trillion more tonnes of carbon dioxide emissions will still carry a 33% probability of exceeding 2 degrees warming, and it will also carry a 1% chance of a rise of four degrees. A recent World Bank report outlined how four degrees would lead to enormous humanitarian and economic costs.

And most seriously, human societies would face an existential threat if we cross tipping points in the Earth system which could lead to irreversible or accelerating impacts. These include destabilisation of the major ice sheets in Greenland and West Antarctica, committing us to a sea-level rise of six meters or more, the thaw of permafrost, releasing huge volumes of greenhouse gases into the atmosphere from the tundra in polar regions, and the gradual disappearance of sea ice, allowing more heat to be absorbed by the ocean.

These tipping points are played down in scientific assessments of climate change because there are many uncertainties. But scientists are recording the evidence that we may be approaching, or have already crossed, a number of them. Arctic sea ice is reducing in area and thickness at an alarming rate. On Greenland, we’ve filmed glaciers the size of Manhattan tumbling into the sea in hours.

But we have lots of the solutions to limit and manage these risks. Many businesses and governments already understand that the transition to low-carbon economic development and growth presents vast opportunities, spurring a new generation of jobs and innovation. Many of the measures to reduce greenhouse gas emissions have multiple economic benefits, such as reducing local air pollution, which kills millions of people around the world each year. So the transition to the low-carbon economy will mean better economic growth, higher living standards and a reduction in poverty.

But we need stronger political leadership. Governments must offer ambitious, equitable and credible national targets ahead of Paris, and reach an agreement that commits countries to increase the strength of their action afterwards to avoid dangerous climate change. This will require two important goals to be reached: zero emissions from fossil fuels by 2050, and zero deforestation by 2030.

As researchers, we have a responsibility to communicate to government, businesses and the public about the scale of the impacts human society faces from unmanaged climate change, and the options we have for dealing with these risks.

The evidence is clear that the world faces profound dangers on our current unsustainable pathway. The alternative low-carbon pathway will significantly reduce the risks of climate change, and help to create greater prosperity and well-being for us, our children and future generations.

Authors: Johan Rockström, Director, Stockholm Resilience Centre; Nicholas Stern, Chair of the Grantham Research Institute on Climate Change and the Environment, London School of Economics; Youba Sokona, Special Advisor on Sustainable Development, South Centre and Future Earth Scientific Committee.

Image: Wildflowers bloom on a hill overlooking a fjord filled with icebergs near the south Greenland town of Narsaq July 27, 2009. REUTERS/Bob Strong