Ten ways the planet could tip into 'Hothouse Earth'

Marlowe HOOD AFP Yahoo News 7 Aug 18;

Paris (AFP) - Even if humanity slashes greenhouse gas emissions in line with Paris climate treaty goals, the planet could overwhelm such efforts and irretrievably tip into a hellish 'hothouse' state, top scientists warned Monday.

Under such a scenario, Earth's average temperature would stabilise 4 or 5 degrees Celsius above pre-industrial levels, rather than the 1.5 C to 2 C (2.7 or 3.6 Fahrenheit) cap called for in the 196-nation pact.

As it is, the world is struggling to curb the manmade carbon pollution that -- with only one degree Celsius of warming so far -- amplifies the likelihood and intensity of deadly heatwaves, droughts and superstorms.

Simply put, climate change continues to outpace the transition to a green, clean global economy.

But that challenge will become exponentially more difficult if Earth itself gets into the act, the researchers reported in the US Proceedings of the National Academy of Sciences (PNAS).

- Tipping points -

No less than ten distinct facets of what scientists call the Earth System could switch from neutral or helpful to harmful, eventually dumping more CO2 and methane into the atmosphere than all human activity combined.

Most have temperature "tipping points" beyond which the release of these planet-warming gases would be irreversible, at least on a human time scale.

"The feedback process becomes self-perpetuating after a critical threshold is crossed," the study said.

"The Earth System may be approaching a planetary threshold that could lock in a continuing, rapid pathway towards much hotter conditions -- Hothouse Earth."

- Weakened carbon 'sinks' -

Earth's forests and oceans have together absorbed more than half of carbon pollution over the last several decades, even as those emissions grew.

But forests are shrinking, and oceans are showing signs of CO2 saturation, according to recent studies.

These carbon sponges or 'sinks', in other words, may be weakening.

- (Not so) permafrost -

Methane and CO2 trapped in the increasingly misnamed permafrost of Russia, Canada and northern Europe is roughly equivalent to 15 years of emissions at today's levels.

The release of these gases -- negligible so far -- would speed global warming and, in effect, hasten their own escape, what scientists call positive feedback.

Similarly, rock-like formations in shallow ocean waters called methane hydrates -- prime suspects for episodes of rapid global warming millions of years ago -- are also vulnerable to global warming, but at what threshold remains unknown.

- Forest 'dieback' -

Global warming of 3 C could condemn 40 percent of the Amazon forests to dieback, a process that would reach well into the next century, according to recent research.

Accidental or land-clearing fires -- not accounted for in these models -- could hasten this destruction.

In Canada, forests that gained CO2-absorbing biomass for most of the 20th century began to lose it around 1970, due mainly to climate-related insect infestations and fires.

Taken together, these forest dieoffs would release billions of tons of carbon into the air.

- Less snow = more heat -

Dramatically shrinking polar sea ice, especially in the Arctic, means the deep blue ocean water that takes its place absorbs as much of the Sun's radiative force -- about 80 percent -- as was reflected back into space by snow's mirror-like surface.

The Arctic's will likely see its first ice-free summer before mid-century, and -- in a 2C world -- could be that way one-in-four years.

Over the last four decades, minimum sea ice extent has dropped by about 40 percent.

- Ice sheets, sea level -

Experts disagree on how much global warming it will take to condemn the West Antarctic and Greenland ice sheets and how quickly they would melt, but all agree that such a tipping point exists, with estimates ranging from 1 C to 3 C.

The consequences for humankind would be catastrophic: Two-thirds of the world's megacities are less than 10 metres about sea level, and is much of the agricultural land that feeds them.

Together, West Antarctica's and Greenland's frozen reservoirs would lift ocean by 13 metres.

Another 12 metres of potential sea level rise is locked in parts of the East Antarctic Ice Sheet that are far more susceptible to climate change than once thought.

- Cascading dominoes -

All of these processes are interconnected, the authors note, and the collapse of one could trigger another.

"The risk of tipping cascades could be significant at a 2 C temperature rise, and could increase sharply beyond that point."

"This cascade of events may tip the entire Earth system into a new mode of operation," said co-author Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research. The "carrying capacity" of a 4 C or 5 C degree world, he has said previously, could drop to a billion people.

Climate change: 'Hothouse Earth' risks even if CO2 emissions slashed
Matt McGrath BBC 7 Aug 18;

It may sound like the title of a low budget sci-fi movie, but for planetary scientists, "Hothouse Earth" is a deadly serious concept.

Researchers believe we could soon cross a threshold leading to boiling hot temperatures and towering seas in the centuries to come.

Even if countries succeed in meeting their CO2 targets, we could still lurch on to this "irreversible pathway".

Their study shows it could happen if global temperatures rise by 2C.

An international team of climate researchers, writing in the journal, Proceedings of the National Academy of Sciences, says the warming expected in the next few decades could turn some of the Earth's natural forces - that currently protect us - into our enemies.

Each year the Earth's forests, oceans and land soak up about 4.5 billion tonnes of carbon that would otherwise end up in our atmosphere adding to temperatures.

But as the world experiences warming, these carbon sinks could become sources of carbon and make the problems of climate change significantly worse.

So whether it is the permafrost in northern latitudes that now holds millions of tonnes of warming gases, or the Amazon rainforest, the fear is that the closer we get to 2 degrees of warming above pre-industrial levels, the greater the chances that these natural allies will spew out more carbon than they currently now take in.

Back in 2015, governments of the world committed themselves to keeping temperature rises well below 2 degrees, and to strive to keep them under 1.5. According to the authors, the current plans to cut carbon may not be enough if their analysis is correct.

"What we are saying is that when we reach 2 degrees of warming, we may be at a point where we hand over the control mechanism to Planet Earth herself," co-author Prof Johan Rockström, from the Stockholm Resilience Centre, told BBC News.

"We are the ones in control right now, but once we go past 2 degrees, we see that the Earth system tips over from being a friend to a foe. We totally hand over our fate to an Earth system that starts rolling out of equilibrium."

Currently, global temperatures have risen about 1 degree above pre-industrial levels and they are rising by around 0.17C per decade.

In their new study the authors looked at 10 natural systems, which they term "feedback processes".

Right now, these help humanity to avoid the worst impacts of carbon and temperature rises, and include forests, Arctic sea-ice, and methane hydrates on the ocean floor.

The worry is that if one of these systems tips over and starts pushing large amounts of CO2 into the atmosphere, the rest could follow like a row of dominoes.

What exactly is a Hothouse Earth scenario?

In short, it's not good.

According to the research paper, crossing into a Hothouse Earth period would see a higher global temperature than at any time in the past 1.2 million years.

The climate might stabilise with 4-5 degrees C of warming above the pre-industrial age. Thanks to the melting of ice sheets, the seas could be 10-60 metres higher than now.

Essentially, this would mean that some parts of the Earth would become uninhabitable.

The impacts would be "massive, sometimes abrupt and undoubtedly disruptive," say the authors.

The only upside, if you can call it that, is that the worst impacts may not be felt for a century or two. The downside is that we wouldn't really be able to do anything about it, once it starts.

Are the current heatwaves in the UK and Europe evidence of a Hothouse Earth?
The authors say the extreme weather events we are seeing right now around the world cannot be immediately associated with the risk of passing 2 degrees C.

However, they argue that it may be evidence that the Earth is more sensitive to warming than previously thought.

"One should learn from these extreme events and take these as a piece of evidence that we should be even more cautious," said Prof Rockström.

"It may support the conclusion that if this can happen at one degree, then we should at least not be surprised or too dismissive of conclusions that things can happen more abruptly than we previously thought."

Surely we've known about these risks before?
What these authors are saying is that up to now, we've underestimated the power and sensitivity of natural systems.

People have been thinking that climate change would be a global emergency for everyone if temperatures rose 3-4 degrees by the end of this century.

But this paper argues that beyond 2 degrees, there is a significant risk of turning natural systems - that presently help keep temperatures down - into massive sources of carbon that would put us on an "irreversible pathway" to a world that is 4-5 degrees warmer than before the industrial revolution.

Any good news here at all?

Surprisingly, yes!

We can avoid the hothouse scenario but it's going to take a fundamental re-adjustment of our relationship with the planet.

"Climate and other global changes show us that we humans are impacting the Earth system at the global level. This means that we as a global community can also manage our relationship with the system to influence future planetary conditions.

"This study identifies some of the levers that can be used to do so," says co-author Katherine Richardson from the University of Copenhagen.

So not only are we going to have to stop burning fossil fuels by the middle of this century, we are going to have to get very busy with planting trees, protecting forests, working out how to block the Sun's rays and developing machines to suck carbon out of the air.

Removing carbon dioxide from the atmosphere, as in this model, will be necessary, say the authors
The authors say a total re-orientation of human values, equity, behaviour and technologies is required. We must all become stewards of the Earth.

What do other scientists say?
Some say the authors of this paper are too extreme. Many others say their conclusions are sound.

"As a result of human impacts on climate, the new paper argues that we've gone beyond any chance of the Earth cooling 'of its own accord'," said Dr Phil Williamson from the University of East Anglia, UK.

"Together these effects could add an extra half a degree Celsius by the end of the century to the warming that we are directly responsible for ‒ thereby crossing thresholds and tipping points that seem likely to occur around 2 degrees C, and committing the planet to irreversible further change, as Hothouse Earth."

Others are concerned that the authors' faith in humanity to grasp the serious nature of the problem is misplaced.

"Given the evidence of human history, this would seem a naive hope," said Prof Chris Rapley, from University College London.

"At a time of the widespread rise of right-wing populism, with its associated rejection of the messages of those perceived as 'cosmopolitan elites' and specific denial of climate change as an issue, the likelihood that the combination of factors necessary to allow humanity to navigate the planet to an acceptable 'intermediate state' must surely be close to zero."


Domino-effect of climate events could push Earth into a ‘hothouse’ state
Leading scientists warn that passing such a point would make efforts to reduce emissions increasingly futile
Jonathan Watts The Guardian 6 Aug 18;

A domino-like cascade of melting ice, warming seas, shifting currents and dying forests could tilt the Earth into a “hothouse” state beyond which human efforts to reduce emissions will be increasingly futile, a group of leading climate scientists has warned.

This grim prospect is sketched out in a journal paper that considers the combined consequences of 10 climate change processes, including the release of methane trapped in Siberian permafrost and the impact of melting ice in Greenland on the Antarctic.

The authors of the essay, published in Proceedings of the National Academy of Sciences, stress their analysis is not conclusive, but warn the Paris commitment to keep warming at 2C above pre-industrial levels may not be enough to “park” the planet’s climate at a stable temperature.

They warn that the hothouse trajectory “would almost certainly flood deltaic environments, increase the risk of damage from coastal storms, and eliminate coral reefs (and all of the benefits that they provide for societies) by the end of this century or earlier.”

“I do hope we are wrong, but as scientists we have a responsibility to explore whether this is real,” said Johan Rockström, executive director of the Stockholm Resilience Centre. “We need to know now. It’s so urgent. This is one of the most existential questions in science.”

Rockström and his co-authors are among the world’s leading authorities on positive feedback loops, by which warming temperatures release new sources of greenhouse gases or destroy the Earth’s ability to absorb carbon or reflect heat.

Their new paper asks whether the planet’s temperature can stabilise at 2C or whether it will gravitate towards a more extreme state. The authors attempt to assess whether warming can be halted or whether it will tip towards a “hothouse” world that is 4C warmer than pre-industrial times and far less supportive of human life.

Katherine Richardson from the University of Copenhagen, one of the authors, said the paper showed that climate action was not just a case of turning the knob on emissions, but of understanding how various factors interact at a global level.

“We note that the Earth has never in its history had a quasi-stable state that is around 2C warmer than the preindustrial and suggest that there is substantial risk that the system, itself, will ‘want’ to continue warming because of all of these other processes – even if we stop emissions,” she said. “This implies not only reducing emissions but much more.”

New feedback loops are still being discovered. A separate paper published in PNAS reveals that increased rainfall – a symptom of climate change in some regions - is making it harder for forest soils to trap greenhouse gases such as methane.

Previous studies have shown that weakening carbon sinks will add 0.25C, forest dieback will add 0.11C, permafrost thaw will add 0.9C and increased bacterial respiration will add 0.02C. The authors of the new paper also look at the loss of methane hydrates from the ocean floor and the reduction of snow and ice cover at the poles.

Rockström says there are huge gaps in data and knowledge about how one process might amplify another. Contrary to the Gaia theory, which suggests the Earth has a self-righting tendency, he says the feedbacks could push the planet to a more extreme state.

As an example, the authors say the loss of Greenland ice could disrupt the Gulf Stream ocean current, which would raise sea levels and accumulate heat in the Southern Ocean, which would in turn accelerate ice loss from the east Antarctic. Concerns about this possibility were heightened earlier this year by reports that the Gulf Stream was at its weakest level in 1,600 years.

Currently, global average temperatures are just over 1C above pre-industrial levels and rising at 0.17C per decade. The Paris climate agreement set actions to keep warming limited to 1.5C-2C by the end of the century, but the authors warn more drastic action may be necessary.

“The heatwave we now have in Europe is not something that was expected with just 1C of warming,” Rockström said. “Several positive feedback loops are already in operation, but they are still weak. We need studies to show when they might cause a runaway effect.

Another climate scientist – who was not involved in the paper – emphasised the document aimed to raise questions rather than prove a theory. “It’s rather selective, but not outlandish,” said Prof Martin Siegert, co-director of the Grantham Institute. “Threshold and tipping points have been discussed previously, but to state that 2C is a threshold we can’t pull back from is new, I think. I’m not sure what ‘evidence’ there is for this – or indeed whether there can be until we experience it.”

Rockström said the question needed asking. “We could end up delivering the Paris agreement and keep to 2C of warming, but then face an ugly surprise if the system starts to slip away,” he said. “We don’t say this will definitely happen. We just list all the disruptive events and come up with plausible occurrences … 50 years ago, this would be dismissed as alarmist, but now scientists have become really worried.”

“In the context of the summer of 2018, this is definitely not a case of crying wolf, raising a false alarm: the wolves are now in sight,” said Dr Phil Williamson, a climate researcher at the University of East Anglia. “The authors argue that we need to be much more proactive in that regard, not just ending greenhouse gas emissions as rapidly as possible, but also building resilience in the context of complex Earth system processes that we might not fully understand until it is too late.”

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