Good news: Some climate change impacts are 'reversible.' Here's what that means

The latest UN climate report this week raised the alarm over the "irreversible" impacts of climate change, such as rising seas and coastal flooding that we will continue to experience for centuries or longer even if we stop emitting greenhouse gases and halt global warming now.

"We are now committed to some aspects of climate change, some of which are irreversible for hundreds to thousands of years," said Tamsin Edwards, a climate scientist at King's College London and co-author of theIntergovernmental Panel on Climate Change (IPCC) report released Monday.

The good news is that some impacts, such the warming of the Earth's surface, can be reversed by removing carbon from the atmosphere at least in theory.

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The Intergovernmental Panel on Climate Change says the world is dangerously close to runaway warming and that humans are "unequivocally" to blame.

What makes some impacts irreversible over others? What does that mean for the impact ofcutting emissions and removing carbon? And whatdoes itultimately mean for the future?Here's a closer look.

What kinds of long-term, irreversible harm does the report talk about?

The report talks about a number of changes that arealready happening and will continue to happenfor centuries,"even if emission of CO2 were stopped immediately." These include:

• A loss of carbon stored in permafrost into the atmosphere.
• An increase in ocean temperatures.
• An increase in global sea level, which is linked to coastal flooding.

Some changesexpected to be irreversible on an even longer timescale up to thousands of years include:

• The melting of the Greenland ice sheet.
• The melting of the West Antarcticice sheet.
• Acidification and loss of oxygen in the deeper parts of the ocean.

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So waitthis means some climate change impacts are reversible?

Yes, in theory.

Many of the scenarios aimed at reaching theParis Agreement target of less than 1.5 C of warming by 2100 involve a concept called "overshoot."

Overshoot acknowledgesthat theaverage globaltemperature will rise above 1.5 C temporarily, due to our inability to reduce emissions fast enough.Once we finally stop emitting CO2, it will stabilize at a temperaturehigher than 1.5 C,said Damon Matthews, a professor and research chair in climate science and sustainability atConcordiaUniversity.

"And then correspondingly, if we are able to extract CO2 from the atmosphere,that has the potential to draw temperatures down from that peak level."

The IPCC report saysthere's high confidence that human intervention has the potential to remove CO2 from the atmosphere and store it in reservoirs for the long term, a technology known as carbon capture and storage.

"The idea of carbon dioxide removal is now much more accepted than it has been previously as something that might be possible to achieve," Matthews said.

Global temperature changes aren't naturally reversible, he said, which may be why many of us may have previously had the impression that, generally, climate change was irreversible.

So which changes are reversible? And how is that possible?

Both surface temperature and acidification of the ocean's surface(but not the deep ocean) are expected to be reversible, the IPCC report said, noting that "other climate changes would continue in their current direction for decades to millennia."

Thereversiblechanges are directly related to the amount of CO2 in the atmosphere, andrespond"relatively quickly" to changes in that amount, Matthews said.

Some of the current warming is due to methane, a greenhouse gas that is much more short-lived than CO2 albeit more powerful in terms of trapping heat. It has a lifetime of just 12 years, compared to hundreds of years for CO2.

"So the amount of warming caused by methane, for example, is much more reversible than the amount of warming caused by CO2," Matthews said. "And if we are able to decrease emissions of methane very dramatically, that also has a potential to ultimately reverse some of the warming that has been caused previously by methane emissions."

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But don't get too excited yet.

The global temperature will stabilize when westop adding CO2 to the atmosphere a point called "net zero"emissions, where any remaining emissions are "cancelled out" by carbon capture or removal.

But Matthews said it willtake muchmore time to remove CO2 already in the atmosphere, enough tolower the global temperature.

"This will benefit future generations, potentially," he said. "Almost nothing is reversible on the timescale of a few decades."

Why are some climate impacts less reversible?

"Any processes that involve the ocean or glaciers or permafrost are processes that have a much longer timescale associated with themmuch more inertia," explained Matthews.

A lot of interdependent slow processes contribute to that, in addition to the fact that water expands at higher temperatures, saidGreg Flato, a senior research scientist with Environment and Climate Change Canada and vice-chair of the IPCC group that authored the report.

"The sea level will continue to rise because large ice sheets, especially, take a long time to come into equilibrium with a new temperature," he said."And the deep ocean takes a long time to warm up to a level that is consistent with the surface."

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The temperature of the ocean is "still responding to CO2 that we put into the atmosphere decades ago," said Matthews, and hasn't yet had time to react to more recent emissions, which it will gradually do in coming decades or centuries.

What will the impact of these irreversible changes ultimately be?

Patricia Manuel, a professor in the School of Planning at Dalhousie University in Halifax, has been thinking about that for a long time. She's been studying how coastal communities need to plan for climate change and its impacts, particularly higher sea levels combined with more extreme weather and the flooding that will come with that.

"Greenhouse gas emissions have more or less committed us to a very long-term sea level rise," she said. "We need to plan for that."

That also means coming to terms with the permanence and irreversibility of it, she said. Land that people have relied on for centuries or more and is tied to their history will disappear.

"It's a real mental shift those shores aren't going to exist in that location much longer," said Manuel."It's a disturbing thing to think about, to know you're leaving behind potentially things that are very dear to you culturally and economically."

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Because such changes are slower, in theory, they should be easier to plan andadapt for, by moving infrastructure such as roads and dikes back from the coast, stabilizing dunes and adding wetlands and oyster reefs for protection.

But in practice, she said, that kind of planning is difficult. "We've already seen how difficult it is because we're still not doing it."

Meanwhile, the rate of sea level rise is ramping up.

Will reducing emissions have an impact on irreversible, long-term harms, like rising sea levels?

Yes, absolutely.

While a certain amount of change is already locked in for the long-term, any change will happen more quickly with increased emissions.

"The rate of sea level rise will depend on future emissions," said Flato.

By 2100, sea level will likely rise 0.28 to0.55 metres if greenhouse gas emissions are cut dramatically.But if emissions remain high, sealevel will likelyrise 0.98to 1.88 metres by the end of thiscentury.And in that case asea level rise of twometres by 2100 and 5 metres by2150 can't be ruled out, the IPCC report said.

Over the next 2,000 years, the global mean sea level will rise by abouttwoto three metres if warming is limited to 1.5 C, two to six metres if limited to 2 C, and 19 to 22 metres with 5 C of warming. And it will continue to rise over subsequent millennia.

Similarly, irreversible changes to ice sheets, glaciers, permafrost and the deep ocean will be faster and more extreme with higher emissions.

That's also the case with theoretically reversibleclimate changes related to temperatures, such as extreme heat, drought andwildfires.

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"If we don't succeed in stabilizing temperatures, it's not going to matter if [climate changes] were reversible or not," said Matthews."Because we will be in a new climate regime and we'll be having to deal with very dramatic and potentially catastrophic climate impacts."

Reaching net zero emissions to stabilize the temperature is key, he said.

"That will be the first thing that determines what level of impacts we will experience over the coming decades."