Our Final Warning: Six Degrees of Climate Emergency

By Mark Lynas

1 Degree

We are already living in a world one degree warmer than that inhabited by our parents and grandparents. Two degrees Celsius, which will stress human societies and destroy many natural ecosystems such as rainforests and coral reefs, looms on the near horizon. At three degrees I now believe that the stability of human civilisation will be seriously imperilled, while at four degrees a full-scale global collapse of human societies is probable, accompanied by a mass extinction of the biosphere that will be the worst on Earth for tens or even hundreds of millions of years. By five degrees we will see massive positive feedbacks coming into play, driving further warming and climate impacts so extreme that they will leave most of the globe biologically uninhabitable, with humans reduced to a precarious existence in small refuges. At six degrees we risk triggering a runaway warming process that could render the biosphere completely extinct and for ever destroy the capacity of this planet to support life.

If we stay on the current business-as-usual trajectory, we could see two degrees as soon as the early 2030s, three degrees around mid-century, and four degrees by 2075 or so.

We could be in for five or even six degrees by the century’s end.

Humanity has returned atmospheric CO2 concentrations to levels last seen in the Pliocene, around three to five million years ago.

Today’s temperatures are already high enough to eventually melt the majority of the Greenland ice sheet and deliver a multi-metre rise in sea levels.

In late December 2015 at the North Pole itself temperatures rose close to freezing. Under normal circumstances, air temperatures at the pole would be averaging around -30°C during the frigid polar nights of December.

Scientists studying the event called it ‘unheard of’ and ‘super-extreme’ – but the following December it happened again.

What happens in the Arctic doesn't stay in the Arctic. By upsetting the energy balance of the planet we are changing the temperature gradient between the equator and the pole. This in turn sets in motion major reorganisations of the flow patterns of the atmosphere and ocean. The consequences are emerging and they are disruptive, and likely to become even more profoundly so.

The melt rate on mountain glaciers around the world is now so rapid that, combined, they now add nearly as much to sea level rise each year as the whole Greenland ice sheet, totalling an estimated 335 billion tonnes of melted ice.

On 25 July 2019 a new UK all-time high-temperature record of 38.7°C was set at Cambridge University Botanic Garden, beating the previous record – set in 2003 – of 38.5°C.

This is what one degree of global warming looks like.

Coral bleaching was virtually unknown in the world’s oceans before the mid-1980s.

Bleaching events are now arriving too frequently for the reefs to recover anything like their previous diversity before the next event.

The latest studies now show that marine heatwaves are becoming sufficiently intense to skip the bleaching process altogether and kill coral organisms directly. The exposed reefs, left vacant by the dead coral polyps, are draped with a layer of algae within a few days, and later begin to dissolve.

The extinction clock is ticking. The final demise of the reefs may come sooner than even many scientists expect because corals are increasingly failing to reproduce at all.

Recent work on the Great Barrier Reef found a 90% collapse in the reproductive capacity of the corals.

Already, at one degree, our globe is becoming impoverished and reduced. We might all weep for what we have done.

2 Degrees

No one knows exactly when it will happen, but at some point in the two-degree world we will see the complete disappearance of Arctic sea ice, resulting in an ice-free North Pole for the first time for about three million years.

Even during previous interglacials, warm periods between successive ice ages when the Sahara flourished with lakes and wetlands, there was still permanent ice in the Arctic. The date that we lose it, Day Zero in the Arctic, will surely be a global warming marker like no other.

The disappearance of the Arctic sea ice could play havoc with the world’s weather.

The Arctic meltdown will also directly affect human populations.

Nearly four million people and 70% of current built infrastructure is located on permafrost zones that will thaw in the two-degree world.

Perhaps the greatest threat posed by permafrost thaw is that it will further accelerate the breakdown of the world’s climate.

In the two-degree world, one study projects that enough of the Arctic permafrost will melt to release 60–70 billion tonnes of additional carbon into the atmosphere.

There is a substantial difference between the area of permafrost that melts in a 1.5°C-warmer world as opposed to two degrees.

The Arctic permafrost carbon feedback is a shortcut to the three-degree world.

It is not just the Arctic. The Antarctic sea ice is in what scientists call ‘precipitous’ decline, with ‘decreases at rates far exceeding the rates seen in the Arctic’,

A potentially disastrous Antarctic tipping point may lie somewhere in the two-degree world.

Warmer oceans would then penetrate hundreds of kilometres into the Antarctic interior, fragmenting the entire continental ice sheet and ultimately delivering more than five metres of sea-level rise. None of today’s coastal megacities could survive such a huge boost to global sea levels

Five metres would require a large-scale relocation of humanity inland, involving upwards of a billion people.

The IPCC projects that 79 million people will be displaced as their homes and communities are inundated in a two degrees world

As soon as 2050, large parts of the world’s coastlines will be experiencing what was previously a once-in-a-century flood event every single year.

At least 136 megacities are at risk of being at least partially flooded in a two-degree scenario,

The gradually rising list of abandoned areas will not just include coastal portions of larger nations such as China and the US. Entire countries will face being submerged or eroded away

Their residents will eventually have to join the millions of climate refugees all over the world who will be looking for a new home.

One study published in 2018 estimated that limiting global temperature increases to 1.5°C would potentially avoid half a million dengue cases in Latin America by the end of the century as compared with a rise of two degrees.

And dengue is only one of a number of insect-borne diseases that might spread as a result of rising temperatures.

The greatest human health risk of all in a two-degree world does not involve transmissible disease, however. It is the risk that hundreds of millions – or even billions – of people will run short of food.

A recent paper in the medical journal The Lancet projected that global temperature rises of about two degrees would lead to a ‘relative reduction of global food availability in 2050 of 99kcal per person per day’

At least half a million people will die from malnutrition in a two-degree world because of climate change. This half-million projected death toll could be a significant underestimate.

Globally, over 40% of the world’s 44 largest megacities will see dangerously high heat conditions each year with 1.5°C of warming, exposing 350 million more urban residents to ‘deadly heat’ by 2050.

Overall, 90 million people in Europe would be exposed to historically unprecedented summer heatwaves in a 1.5°C warmer world. This number rises to 163 million – twice the population of Germany – in a two-degree scenario.

The most recent aggregate study found that more than two billion people would be exposed to extreme heatwaves at least once every 20 years in a two-degree world.

Two degrees of global warming will raise the height of the freezing level in the Peruvian Andes by about 230 metres, eliminating most of the smaller icefields altogether

The global picture is just as dire. Under current rates of ice loss, most of today’s glacier volume will vanish in the Caucasus, the European Alps, the tropics, North America and New Zealand by the second half of the century.

The greatest concern, however, lies in South Asia, where around 900 million people depend on waters supplied by the rivers which rise in the high mountains of the Karakoram–Himalayas.

(The so-called ‘wettest place in the world’, Mawsynram in Meghalaya state in north-east India, receives an incredible 12 metres of rainfall annually.)

However, in a two-degree future the forecast is for a widespread intensification of the monsoon – not just in South Asia, but also for other monsoonal systems across the world.

Two degrees of warming means that 146% more land area and 149% more people will be exposed to extreme five-day precipitation events.

In 2019 the strongest monsoon for a quarter of a century killed 1,600 people across India.

According to a major 2018 study, a fifth of the world’s land area will see a significant increase in severe week-long flooding in the two-degree world,

Perhaps the greatest danger, however, is drought.

Globally, 410 million more people will be exposed to severe drought conditions in the two-degree world. For them, this future means long queues for water tankers, the drying out of rivers, streams, lakes and wells, and the failure of harvests.

Perhaps the most worrying drought hotspot is the Amazon rainforest.

The decades-long conflagration of the Amazon rainforest will probably release somewhere between 25 and 55 billion tonnes of carbon into the atmosphere.

Amazonia also stores immense quantities of peat, up to seven metres thick in places.

Once the trees fall, it will be exposed to decomposition or to incineration from wildfires. Either way the carbon enters the atmosphere, adding a further boost to global warming.

If we allow it to burn, the additional carbon will help push the planet beyond the guardrails of two degrees, and into an even hotter future.

Unfortunately, one of the planet’s most biodiverse and important ecosystems will not survive two degrees intact.

The Great Barrier Reef suffered back-to-back bleaching events in 2016 and 2017, losing fully half its coral cover in the process.

Even if global temperatures stay under 1.5 degrees, 70–90% of reef-building corals will be lost. With two degrees of warming, this increases to 99%.

Coral reefs around the world are critically endangered in the very imminent future. Two-thirds of the world’s reefs could be gone within just a decade from now.

What is left of coral reef structures will be steadily dissolving across the whole world by mid-century due to ocean acidification.

The glory days of coral will be over, never to return in our lifetimes at least.

While two degrees will stretch many ecosystems to breaking point, three degrees will usher in a wholescale mass extinction.

Human societies can aim to survive the two-degree world in some semblance of their current condition. Another degree, however, will stress our civilisation towards the point of collapse.

3 Degrees

Entering the three-degree world means we are now living in a hotter climate than any experienced on Earth throughout the entire history of our species.

You have to wind the geological clock back about three million years, to an epoch called the Pliocene, to encounter a world where global temperatures averaged 2–3°C higher than at the start of the 20th century.

Sea levels during the Pliocene were as high as 22 metres above where they are today.

Researchers now suggest that we’ll be back in the Pliocene as soon as 2030 with current emissions trends.

Our return to the Pliocene, with its drastically reduced ice sheets, is already well under way.

Our return to the Pliocene, with its drastically reduced ice sheets, is already well under way. half a metre by 2100, this will inundate land currently inhabited by 50 million people.

Either cities adapt, or they will eventually have to be abandoned.

On June 11th 2099, Delhi had just recorded an all-time high temperature of 48°C. The extreme heat of 2019 will be considered an unusually cool summer in the three-degree world.

Hundreds of millions of people will be experiencing ‘dangerous heatwave’ conditions never before seen in today’s climate.

By the time the global average temperature reaches three degrees, half the world’s population will be annually exposed to heatwave conditions that can kill.

The global human population is projected to rise to 10 billion. To feed these extra mouths requires doubling food production globally by mid-century.

In a three-degree scenario we could see food production cut by half. which is nothing less than a recipe for global mass starvation.

Worldwide food shortages are the most likely trigger of large-scale civilisational collapse in a three-degree world

The Brazilian rainforest is currently one of the largest living carbon reserves on the planet, storing 150–200 billion tonnes in its biomass and soils. Losing just half of this gigantic carbon store would be equivalent to a decade of humanity’s fossil fuel emissions.

This spectacular ecosystem, which is thought to have endured on Earth for at least 55 million years, could be largely gone in our lifetimes. After the loss of tropical coral reefs, which will already be ecologically functionally extinct in the three-degree world, the death of the Amazon will stand as the second great ecosystem collapse of planetary significance brought about by global climate heating.

But this epic disaster is not even the largest positive feedback

A far greater store of carbon currently lies locked up in the frozen soils of the Arctic. This could total well over a trillion tonnes.

The key question now is how rapidly it will thaw, over how wide an area, and how much additional carbon will spew into the atmosphere as a result.

Nearly three-quarters of the globe’s inventory of permafrost will be turned to mush as temperatures rise to three degrees.

And it gets worse.

‘A 50-gigatonne reservoir of methane, stored in the form of hydrates, exists on the East Siberian Arctic Shelf. It is likely to be emitted as the seabed warms, either steadily over 50 years or suddenly.’

And that is not all.

Most studies agree that a three-degree world sees permanent summertime ice-free conditions in the Arctic Ocean, perhaps as soon as mid-century.

After coral reefs and the Amazon, this is the third globally significant ecological collapse in our hotter world.

It is clear that entering the three-degree world risks pushing the global heating process beyond humanity’s control.

Maybe it is worth taking a look, therefore, at what the four-degree world has in store.

4 Degrees

In a four-degree world billions of people are experiencing ‘dangerous’ heat index conditions every year, encompassing most of the world’s great megacities

Global exposure to extreme heat increases 30-fold, while in Africa it increases by a factor of more than 100.

Even sub-polar regions will experience 40–80 days of heatwave conditions each year.

The interior of Alaska will see annual temperatures in excess of 35°C (95°F).

This extreme heat will kill increasingly large numbers of people.

Temperatures at four degrees of global heating are high enough to kill anyone, however fit and healthy

At four degrees we are beginning to turn parts of our once-temperate world into a lethal hothouse, hostile to virtually all life.

The refugee numbers could be astronomical, encompassing a significant fraction of the Earth’s people.

We are making our world increasingly unsuitable for life, and there is nowhere else to go.

More than half the world’s land surface will become classed as ‘arid’ in a four-degree scenario.

We will have literally set our world on fire

Millions of heat refugees will be joined by millions more as vast fires sweep across the landscape –

Unless global heating is curbed we are on course to eventually transform the Earth into an entirely ice-free state, for the first time in tens of millions of years and without it there is nothing to keep our planet cool.

The 1998 floods were the worst in Bangladesh’s recent history. In total 30 million people were affected and over a thousand died. A million people were displaced,

Projections indicate a ‘virtually certain’ likelihood of heavier maximum rainfall events across the whole land surface of the planet.

Weeks of downpours will lead to flooding on a scale never before experienced in the current climate.

Four degrees will see temperatures rise above the thermal tolerances of staple crops across virtually all the world’s major food-producing areas.

Four degrees of global warming turns virtually the entire area that produces crops in the US into a Dust Bowl state.

In an average year, by the time global temperatures hit four degrees, nearly two-thirds of the worldwide wheat-producing area will be gripped by drought.

Be in no doubt what this means. The world will run out of food.

The latest research suggests that the last time global temperatures were four degrees higher than now was roughly 15–40 million years ago.

Weather patterns in a four-degree world will be barely recognisable from the ones we are familiar with today.

Planet Earth has now entered the endgame, and four degrees is unlikely to be our final destination. By this stage large-scale feedbacks have kicked in, threatening to turn the heating process into an unstoppable upwards spiral.

By the end of the century we will be knocking on the door of an even hotter era – the super-greenhouse world of five degrees.

5 Degrees

At five degrees, humanity has lost control of global temperatures, which are now spiralling relentlessly upwards. Food production is decimated, and large areas of the planet are too hot for humans to inhabit.

It is almost game over.

How can ten billion people avoid mass starvation when world harvests fail completely?

As I write, 850 million people are malnourished due to lack of food, while the super-rich criss-cross the world in private jets

We all acquiesce in this ongoing moral outrage as if it were the natural state of affairs.

The human population will be drastically reduced; and much of the remaining life on Earth will be eliminated.

Highs into the mid-60s Celsius occur regularly in North Africa in the five-degree world, and rainfall has virtually ceased across many land areas.

At five degrees we are seeing humanity clinging on in only small refuges, surrounded on all sides by spreading deserts, forests in flame and rising seas.

The rest of our planet is a silent cemetery, suitable for the dead but no longer with much to offer the living.

As we enter the five-degree world, we have now made the planet hotter than at any point for over 50 million years.

6 Degrees

No one should doubt that six degrees of global heating would represent a catastrophic failure of the Earth system.

To get any additional insight into the possibilities of a six-degree world, we have to return to the geological and palaeoclimatic literature.

If we want to understand the future, therefore, our best option is to better understand the past.

There is only one warming event over the last 100 million years that comes close to our 21st-century carbon pulse in terms of its abruptness in changing the climate.

This was when a gigantic asteroid crashed into the planet 65 million years ago, causing the world’s most famous mass extinction.

There is only one occasion over the last half-billion years when the planet’s self-regulatory climatic thermostat has come close to failing completely. This event was a true apocalypse, one that wiped out 90% of species and arguably came within a whisker of entirely eradicating life on Earth.

Although the end-Permian wipeout took place 251 million years ago, some of these kill mechanisms seem eerily familiar today.

Unstoppable greenhouse gas emissions once drove a worldwide catastrophe – and could potentially do so again.

Although not as instantaneously dramatic as the later asteroid strike that ended the Cretaceous, the end-Permian mass extinction still ranks as the worst in the entire history of the Earth.

As terrestrial ecosystems were scrubbed from the land in a catastrophic bout of soil erosion, the oceans filled with dead and dying plants and animals.

The oceans deoxygenated right down to the depths to kill off the vast majority of oceanic life.

So just how sudden and rapid was the end-Permian global temperature rise that denuded the land and turned the world’s oceans into stagnant ponds?

The combined efforts of human beings to dig up and burn fossil fuels to power our global industrialised economy is taking place at least ten times faster than the catastrophic carbon release that drove the world’s worst-ever mass extinction.

At no point since complex life appeared on Earth has so much carbon been released as quickly as we are releasing it now.

So what would the end-Permian mass extinction look like in a world that included human civilisation?

Humans will not be the first casualties of the mass extinction – more likely we will be among the last, clinging on until the bitter end.

The question we are facing today is: could humans emit so much carbon so quickly that we trigger a surprise runaway greenhouse and render the biosphere completely extinct?

In refusing to cut back on our carbon emissions, we are imperilling the existence not just of our species but of the entire planet – perhaps the only one in the history of the whole universe that has nurtured and brought forth life in all its magnificent beauty and diversity.

Six degrees sees the greatest mass extinction ever on Earth, greater even than the end-Permian catastrophe that destroyed 90% of species alive at the time. As I write, human carbon emissions are at least ten times more rapid than those that triggered the end-Permian cataclysm.

Over the longer term, heating this extreme raises the prospect of a runaway greenhouse effect that evaporates the oceans and sterilises the biosphere, turning the Earth into Venus a billion years too soon.

If all this sounds overwhelming, remember one thing: we are not yet doomed.

If global emissions ceased tomorrow, the planet would not even warm by 1.5°C.

Anyone taking the evidence I have presented here as a reason to declare that ‘it’s too late’ to change our future is wilfully misinterpreting my message.

We all know what is really required; we must kick the carbon habit, shut off the pumps and leave the remaining fossil fuels in the ground.

Never despair, because there will always be someone whose life it is not yet too late to save. That person might even be your child.

I will never surrender to despair and will always fight to save what still remains, until the heat stops rising and our children have a future.

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