There are no simple solutions to complex problems like climate change. But there have been times in the past when the world has come together to try to fix an environmental crisis.
How did we deal with acid rain, for example, or the hole in the ozone layer? And are there lessons for tackling the bigger issue of global warming?
1970, '80s and '90s: Acid rain
It's the 1980s, and fish are disappearing in rivers across Scandinavia. Trees in parts of the forests are stripped bare of leaves, and in North America some lakes are so devoid of life their waters turn an eerie translucent blue.
The cause: Clouds of sulphur dioxide from coal-burning power plants are travelling long distances in the air and falling back to Earth in the form of acidic rain.
"In the '80s, essentially the message was that this was the largest environmental problem of all time," says Peringe Grennfelt, a Swedish scientist who played a key role in highlighting the dangers of acid rain.
Headlines warning of the threats of acid rain were commonplace. For years there had been obfuscation, denial and diplomatic stand-offs, but once the science was settled beyond doubt, calls for action quickly gathered momentum. It led to international agreements curbing the pollutants from burning fossil fuels that acidify rain.
Amendments to the Clean Air Act in the US saw the development of a cap and trade system, giving companies an incentive to reduce emissions of sulphur and nitrogen, and trade any excess allowances. Each year, the cap was ratcheted down until emissions dropped dramatically.
So did it work? Acid rain is now largely a thing of the past in Europe and North America, although it remains a problem elsewhere, particularly in Asia.
However, Canadian scientist John Smol, a young researcher back in the 1980s, says in many ways acid rain was a "success story", showing that countries can come together and deal with an international problem. "If you don't price pollution, people will pollute. We learned that for sure," he says.
1980s: The ozone hole
In 1985, news of another looming environmental problem hit the headlines. Scientists from the British Antarctic Survey (BAS) alerted the world to a large and expanding hole in the ozone layer above the Antarctic. It was caused by the chlorofluorocarbons - greenhouse gases better known as CFCs - then used in aerosols and refrigerants.
"Suddenly it goes 'boom', and it drops really quickly," says BAS polar scientist Anna Jones, referring to the dramatic thinning of the band of gas that shields the planet from harmful UV rays.
CFC ban reduced global warming, say scientists
Ozone layer 'rescued' from CFC damage
'Ozone hole vigilance still required'
Ozone over the Antarctic had been diminishing since the 1970s, but news the hole now covered the entire Antarctic continent triggered worldwide alarm. In 1987, world leaders signed the landmark Montreal Protocol, hailed as one of the most successful environmental treaties of all time.
Ozone-depleting chemicals were phased out, with industry switching to "CFC-free" aerosol cans that appealed to green consumers. "It was a global problem, but industry, the scientists, the policymakers came together," says Dr Jones.
"They acted quickly; they acted with a mechanism that allowed continual tightening of that protocol. It's a very important template for how you can make things work."
Despite the success of the Montreal Protocol, there have been setbacks. It was discovered that hydrofluorocarbons (HFCs), developed as alternatives to ozone-depleting chemicals, were potent greenhouse gases.
And there was a mysterious spike in CFCs traced to China. Both led to further action. And while the ozone hole is "on the road to recovery", ozone-depleting chemicals stay in the atmosphere for a long time, meaning repair is a long, slow process.
1920s to 2020s: Leaded petrol
For decades we used leaded petrol as fuel - as companies added lead additives to help petrol burn more efficiently. Leaded petrol releases lead particles into vehicle exhausts that can be breathed in, causing a variety of health problems, including heart attacks, strokes and impaired mental development in children.
After a long battle between scientists, regulatory authorities and industry, a consensus around the health risks emerged, and rich nations banned leaded petrol from the 1980s onwards.
Leaded petrol now eradicated worldwide, says UN
Why did we use leaded petrol for so long?
Use in developing countries lingered on, however, owing to the fuel being cheaper to produce than unleaded petrol. Following a long campaign by NGOs, industry groups and governments, under the umbrella of the United Nations Environment Programme (UNEP), the final drop of leaded petrol was pumped into a car's tank only months ago.
And while the world has officially eradicated leaded fuel, lead pollution lingers on in the environment in dust and soil, where it can persist for a long time.
Lessons for climate change?
With climate change dominating the news agenda, we hear very little nowadays about the likes of the ozone hole. Yet, there are parallels between these crises and the monumental one that is climate change.
For a long time, acid rain was a source of international conflict, with some denying its very existence and the fossil fuel industry pitted against environmentalists. Does that sound familiar?
According to Prof Smol, the debates and discussions over acid rain were training for the more complex issues of climate change. "The first lesson I learned was that we had to effectively communicate the results of our studies, not just to other scientists but to policymakers and the public at large," he says.
"If there is an information vacuum, it will be immediately filled by vested interest groups."
Prof Smol says the situation is even more complicated today, with the growth of social media and the spread of misinformation.
When it comes to the international push to eliminate leaded fuel, Rob de Jong, head of UNEP's sustainable mobility unit, says a key lesson was the value of a harmonised approach. "The whole leaded petrol campaign heavily invested in public awareness, heavily invested in social and community action, heavily invested in focusing on the impact this has on children."
And the steps taken by the international community to reduce ozone-depleting chemicals show - on a smaller scale - the kind of co-operation that will be needed to tackle the warming world.
"The climate change problem is much more complicated to solve than the ozone problem because we don't have immediate alternatives to fossil fuels in the way that we had alternatives to CFCs," says Dr Jones. "But, that's not a reason for not doing something - the problem is too important, it's too big and they need to get on with it.
"When industry and governments have come together in the past they have solved a globally-threatening environmental problem - now they need to show they can do it again."
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