Burning the rest of the world's untapped fossil fuel resources - an equivalent five trillion tonnes of carbon emissions - would push up the average global temperature by between 6.4C and 9.5C.
Those are the big figures behind a major new study published today in the journal Nature Climate Change - and they're much more serious than had previously been anticipated.
The study, led by the University of Victoria in Canada, further estimated that this level of extra emissions would elevate Arctic temperatures between 14.7C and 19.5C.
In the absence of efforts to mitigate climate change, cumulative carbon emissions would likely exceed two trillion tonnes of carbon before the end of this century.
The modelling study used simulations from comprehensive climate models to explore the relationship between warming and the total amount of potential carbon emissions, based on estimates of world fossil-fuel reserves.
"Our study shows a profound climate change in the absence of further mitigation," study lead author Katarzyna Tokarska said.
"How the Earth's climate responds to such high levels of emissions adds a new dimension to scientific knowledge."
New Zealand climate scientist Dr James Renwick, of Victoria University of Wellington, wasn't surprised at the new projections.
It had been well-known for some years now that really large emissions of CO2 would lead to significantly large climate changes, he said.
"The relationship appears to be essentially linear - twice as much CO2 emitted from here equals twice as much eventual warming - right out to the exhaustion of most known reserves of fossil fuels."
With continued strong emissions of greenhouse gases, global mean temperature rises are likely to be in the range 2.6C to 4.8C by 2100, relative to 1986 to 2005.
Dr Renwick said even the 2C of total warming that nations were trying to achieve through UN-led negotiations would be difficult to deal with, but the range estimated in the new study would be catastrophic; "perhaps apocalyptic would be a better word".
It would mean tens of metres of sea level rise and would change rainfalls patterns and temperature extremes so much that current patterns of global food production would be completely destroyed, or very badly damaged at least, he said.
The study comes soon after carbon dioxide levels in the atmosphere exceeded the symbolic level of 400 parts per million for the first time in the Southern Hemisphere, and also follows fresh data showing New Zealand's greenhouse gases rose by one per cent in 2014 and by 23 per cent since 1990.
New Zealand has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.
Victoria University's Dr James Renwick answered these questions from the Herald about the new study's findings.
Q. Do these projections surprise you for any reason?
A. No they don't.
They are in line with results included in the last report by the United Nations' Intergovernmental Panel on Climate Change (IPCC), as the authors note.
It has been well-known for some years now that really large emissions of CO2 will lead to really large climate changes.
The relationship appears to be essentially linear - twice as much CO2 emitted from here equals twice as much eventual warming - right out to the exhaustion of most known
reserves of fossil fuels.
Q. What would this scale of warming mean for New Zealand and for the wider planet, compared with the 2C of warming nations are trying to keep to through the UN-led process?
A. Two degrees of warming will be difficult enough to deal with, but this much change would be catastrophic - perhaps apocalyptic would be a better word.
It would lock in tens of metres of sea level rise - eventually 70 to 80 metres - and would change rainfalls patterns and temperature extremes so much that current patterns of global food production would be completely destroyed, or very badly damaged at least.
We are already seeing temperatures of over 50C in India with one degree of global warming.
With an extra 10C of warming, extreme air temperatures of over 60C, maybe even 70C, would be possible.
The resulting hunger and conflict over food and resources would likely result in the death of a significant fraction of the global population - and the extinction of many of the earth's current species of plants and animals.
Q. Is it realistic to talk about this scenario of huge levels of warming, given the belated yet increasing efforts by governments to curb emissions today?
A. It's a very good thing to have in the back of one's mind.
This is what's at stake, if we don't pull back from fossil fuel extraction and burning.
The less the warming is allowed to continue, the less the impacts will be, so the sooner we cap the emissions, the better.
Q. This paper comes shortly after CSIRO scientists confirmed that the baseline reading at Tasmania's Cape Grim station recorded, for the first time in the Southern Hemisphere, that carbon dioxide readings had exceeded the symbolic level of 400 parts per million (ppm). Why is this milestone significant?
A. It is symbolic because it's a "milestone" round number, and it's a level that marks the previous warm period in earth's climate history prior to the last million or so years of ice age cycles.
Around three million years ago was the last time CO2 concentrations were around 400ppm.
At that time we know that temperatures were 2C or so above present-day values, and sea levels were 10 metres, or up to 20 metres, higher than present. So we are on the edge of significant melting of the Greenland and West Antarctic ice sheets.
Q. Over the weekend we also learned that New Zealand's greenhouse gas emissions increased by one per cent in 2014, adding to a 23 per cent increase since 1990. What was your reaction to this and do you feel we are doing enough to slow climate change?
A. These figures are an indictment of New Zealand Government policy, or lack of policy, around climate change mitigation.
The Government will meet its emissions reductions targets by trading in international emissions units, most of which turn out to be of dubious value, while letting actual emissions continue to increase.
While this is legal, it does not help the climate system much.
New Zealand has a huge opportunity to show some global leadership on this issue.
We are in the top bracket of emitters, on a per-capita basis, and as the recent Royal Society of New Zealand report shows, there are many things we can do now that will reduce our emissions.
I am saddened that as a country, we are not doing the right thing.
We are also missing a great economic opportunity, to be at the forefront of change - that's always where the money is made.
Q. Following December's landmark climate conference in Paris, where nearly every country in the world agreed to cut their own emissions, what do you see as the next big moment on the road ahead?
A. There's a "COP" [Conference of the Parties to the United Nations Framework Convention on Climate Change] meeting every year and the next one is later this year.
From here on, every one of these meetings must be taken as an opportunity to monitor what each country is actually doing, and to really encourage every country to up their targets and their ambition.
In terms of CO2, it would be a huge milestone to see atmospheric concentrations stabilise.
Currently they are rising faster than ever recorded before.
To see the rate of rise start to drop, then seeing the actual amount stop increasing, is critical.
The sooner the better.