Earth's average global warming must stay below 1.5degC if we want to avoid catastrophic environmental damage, a new report says.
The Intergovernmental Panel on Climate Change (IPCC)'s special report 'Global Warming of 1.5degC' warns that we need to take unprecedented action to ensure the planet's warming stays manageable.
The report says if Earth warms by 2degC, hundreds of thousands of species could go extinct, natural environments will die and coastal communities will be flooded out of existence.
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Former IPCC lead author Dr Jim Salinger says it's the most significant climate report to date.
"The difference between the impacts of 1.5degC and 2.0degC is earth-shattering. For example, coral reefs would decline by 70 - 90 percent with global warming of 1.5degC, whereas virtually all (>99 percent) would be lost with 2degC.
"Species loss and extinction are projected to be significantly lower at 1.5degC of global warming compared to 2degC."
If global warming continues to increase at the current rate, it's predicted to reach 1.5degC between 2030 and 2052. At that temperature, many species are at risk of extinction - but if the global average temperature rises by another half a degree, about 400,000 species could die out.
The report says global temperatures have already risen by 1degC higher than average warming increases prior to industrialisation. Earth is now warming at about 0.2degC per decade, which lead report author Associate Professor Bronwyn Hayward from the University of Canterbury says could have a massive impact.
"This doesn't sound like much but if we carry on like this, effectively the world will be warmed 50 percent more that it has already experienced, between 2040 and 2050, and some regions will feel the effects of these changes even more severely and quickly.
She says the findings "mark the end of 'magical thinking' about climate change".
"The report is unequivocal, our climate is changing now," she says.
"Without unprecedented cuts to emissions now, we will have fewer opportunities to develop sustainably and will be required to rely increasingly on unproven, risky and possibly socially undesirable technologies to remove carbon from the atmosphere in the future."
Professor Hayward says the report is groundbreaking because it places climate change within a social context and asks how humanity can make urgent changes to limit its devastation.
"But to avoid climate warming above 1.5C, we have to scale up action in unprecedented ways across all sectors of our economy and everyday life, over the next 10 years."
David Tong from the World Wildlife Fund (WWF) says the report's release is a landmark moment that shows humans have the power to prevent the disastrous effects of climate change.
The report says human activity is estimated to have caused about 1degC of global warming above pre-industrial levels.
Mr Tong says the findings have particular significance for islands like Kiribati.
"The difference between 1.5degC and 2degC is huge. For our Pacific neighbours, it's an existential difference."
Professor Hayward says a future warmer than 1.5degC will put New Zealand's own coastal communities in jeopardy, providing us with another reason to avoid it.
"If the world's temperatures remain at or below 1.5C the report suggests we could avoid an additional 10 cm of sea level rise on average, over and above the trajectories we are already committed to by 2100."
The report also has enormous repercussions for icy land masses like Antarctica and Greenland, as warming of 1.5degC to 2degC could trigger rapid melting and sea level rises.
Mr Tong says he's confident New Zealand can work to keep warming below the temperature danger zone.
"This is a challenge but New Zealanders have a history of doing incredible things in the face of big global challenges. By acting now, our businesses, communities and every single one of us can reap the benefits of being a first mover in the switch to a new clean energy economy."
Professor James Renwick from Victoria University of Wellington says the report makes for "sobering reading".
"We are currently living with 1degC of global warming and we're seeing effects already in extreme events and impacts on ecosystems and societies worldwide.
"More warming, even half a degree, means more and bigger impacts, but it is clear that a 1.5degC world would be a lot more manageable and recognisable than a 2degC world."
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He says while it's physically possible to prevent warming from increasing above 1.5degC, it would require "huge political and social commitment" from every country on Earth.
"The world would need to reach zero carbon by 2050, with a 50 percent reduction between now and 2030. Given the past trajectory of global emissions of greenhouse gases (nearly a doubling in the past 30 years), this represents a truly heroic and unprecedented effort to turn the global economy around. Are the governments, the businesses, and the people of the world up to it?"
Lobby group Coal Action Network says the findings "should be ringing in the ears" of Climate Change Minister James Shaw as he negotiates the Zero Carbon Act with the Opposition.
"He cannot cave to special interests over science," says spokesperson Cindy Baxter. "This report should bolster his case for strong and early action."
Greenpeace hopes the report will "shock the world" into immediate action.
"We have 10 years to save the world as we know it," says climate and energy campaigner, Amanda Larsson.
"This really is our last chance to avoid catastrophic climate change, the effects of which will be felt in our lifetime. We need to pull out all the stops to fast-track clean energy, especially solar, and to clean up our dairy industry through a transition to regenerative farming. We have to act now, and very quickly."
She says New Zealand's new ban on oil and gas permits mean we're emerging as a global climate leader and we must keep up the good work.
"As a small nation, our biggest impact is in showing the world that immediate and dramatic action is not only possible, but it's achievable."