Lord Stern: we need negative emissions to avoid 2C warming #auspol

Speaking at the Royal Society, top climate economist reflects on challenges and opportunities a decade after his seminal review into implications of a warming world.

I am going to speak about five issues. First, I will outline the risks, the required action and the global agenda.
Next, I will speak about the urgency and scale of action required.
Third, I will describe the 21st century growth story, and how to deliver on the global agenda.
Then I will turn to the importance of building sustainable infrastructure.
And finally I will look forward to the next ten years and the prospects for the future.
Let us begin by considering where we may be headed on our current pathway in terms of atmospheric concentrations of greenhouse gases and global average temperatures.
It is worth remembering just how robust the science of climate change is, built on two centuries of theory and evidence since Joseph Fourier first observed that the Earth is warmer than it otherwise would be without its atmosphere.
Emissions of carbon dioxide and other greenhouse gases from the burning of fossil fuels and other human activities increase concentrations in the atmosphere, trapping more infra-red radiation around the Earth, leading to the rise in global mean surface temperature that has been recorded instrumentally since the mid-19th century.
Importantly, the evidence has been growing ever stronger that the risks of unmitigated climate change are immense.
Current annual emissions of greenhouse gases are about 50 billion tonnes of carbon-dioxide-equivalent, compared with about 41 billion tonnes in 2005, so we are still on an upward trend.
Atmospheric concentrations of greenhouse gases are rising rapidly, and have now reached around 450 parts per million of carbon-dioxide-equivalent.
We are adding to greenhouse gas levels by more than 2.5 parts per million of carbon-dioxide-equivalent every year, and that rate is likely to accelerate with little or weak action to reduce emissions.
This rate has risen from about 0.5 parts per million of carbon-dioxide-equivalent per year between 1930 and 1950, one part per million from 1950 to 1970, and 2 parts per million between 1970 and 1990.
Inaction or weak action over the rest of the century, such that global emissions follow the equivalent of the high emissions pathways considered by the Intergovernmental Panel on Climate Change for instance, could take us to well above 850 parts per million of carbon-dioxide-equivalent by 2100.
That would result in the possibility of global mean surface temperature reaching more than 4°C or 5°C above its level in the second half of the 19th century.
Such rises in temperature would create risks that are unprecedented for humankind.
The potential damage from climate change intensifies as the world gets warmer.
Annual global mean surface temperature is already close to 1°C higher than its pre-industrial level, and some months of this year were more than 1.5°C above pre-industrial level.
Hence we are approaching the edge of the range of global mean temperature that has existed over the past 10,000 years, known as the Holocene epoch, during which human civilisation has developed.
The stable climate provided the conditions necessary for the cultivation of cereals, which allowed the formation of villages, and the possibility of surpluses, leading to trade.
We are already seeing the impacts around the world today associated with only 1°C of warming, with shifts in precipitation, the decline of glaciers and ice sheets, and rising sea levels.
Yet these are comparatively small compared with the risks we would face in the future from unmitigated climate change.
There is strong and growing evidence that a rise in global mean surface temperature of more than 1.5°C above pre-industrial level would have very serious consequences, and an increase of more than 2°C would carry still higher risks.
For instance, according to the most recent assessment by the Intergovernmental Panel on Climate Change, the last interglacial period about 125,000 years ago was probably no more than 2°C warmer than pre-industrial levels, yet global sea levels were about 5 to 10 metres higher than today.
Recent modelling suggests that if the rise in global mean surface temperature is held at 1.5°C, sea level rise would tail off during the next century, whereas 2°C of warming would mean sea level rise continues for many centuries.
That is an indication that even 2°C of warming would eventually lead to a radical redrawing of the world’s coastlines.
While such a sea level rise would not happen immediately, we do not know how quickly the land-based ice sheets in Greenland and Antarctica might destabilise.
Indeed, some researchers think that the destabilisation of large parts of the West Antarctic ice sheet has already become unstoppable.
This is just one example of a so-called tipping point, beyond which impacts accelerate, become unstoppable, or become irreversible.
We do not know where these tipping points lie, but we know that there could be many such examples, including the thawing of the permafrost, leading to the release of carbon dioxide and methane, or the die-back of the Amazon and other tropical rainforests, which would reduce the take-up of atmospheric carbon dioxide.
Such tipping points could have rapid and potentially catastrophic impacts.
And we do not know how many tipping points might lie between today’s climate and a world that is 2°C warmer than pre-industrial level.
And if we go beyond warming of 2°C, to 3°C or more, we will create a climate that has not occurred on Earth for millions of years.
That is far beyond the evolutionary experience of modern Homo sapiens, which have only been around for less than 250,000 years.
Warming of 4°C or 5°C would likely be enormously destructive.
The reasons we live where we do would be drastically changed, usually through too much or too little water, as both floods and droughts increase in different parts of the world, and sea level rises across the globe.
These radical changes would cause the migration of hundreds of millions, perhaps billions, of people, potentially leading to severe and sustained conflict.
That is the future our children, grandchildren and future generations face if we do not act.
It is those risks that helped to focus the attention of governments last year as they considered the creation of a new global agenda.
The milestone events of 2015 have set a new global agenda focused on three simultaneous challenges: re-igniting global growth, delivering the sustainable development goals, and driving strong action on climate change.
At the centre of all three of these challenges lies sustainable infrastructure.
Well-designed infrastructure can be pro-growth, pro-poor, and pro-climate.
But it must be delivered with much greater urgency and scale.
Delay is dangerous.

Press link for more: Climate change news

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