When world leaders meet in Dubai this November for the 28th UN climate conference (COP28), it will mark eight years since the Paris Agreement came into being.
Back then, most of the world's nations responded to scientific warnings that allowing the planet to heat by more than 1.5 degrees Celsius (2.7 Fahrenheit) would expose millions more people to catastrophic heat waves and intensifying storms and wildfires.
They did so by agreeing to hold global temperature rise to well below 2 degrees Celsius (3.6 Fahrenheit) compared to pre-industrial levels and "pursue efforts" to keep it below 1.5 degrees.
But why that specific number, and what happens if we pass that threshold?
1.5 degrees Celsius as a line of defense
Scientists recommended the 1.5 Celsius limit as a kind of line of defense. Sticking to this target, experts say, would mean a better chance of avoiding the more extreme and irreversible climate effects that are very likely at higher levels of warming.
Johan Rockström is director of the Potsdam Institute for Climate Impact Research (PIK) in Germany and coauthor of a study that looked at the effects of global temperatures rising beyond 1.5 C.
Speaking in a video for the Stockholm Resilience Centre, he described the 1.5 C limit as not "some arbitrary negotiated number in the Paris Accord" but something that cannot be negotiated.
"It is a level we really need to try and hold as far as we ever can," he said.
But to keep to that limit, says the United Nations, current global emissions need to be halved by 2030, which is less than seven years away.
How close are we to 1.5 degrees?
Global temperatures have been rising by an average of 0.08 Celsius per decade since 1880. That rate started speeding up in 1981 and has, since then, more than doubled.
The 10 warmest years on record were all after 2010. Now climate scientists are predicting that 2023 will be the hottest year ever recorded with a global average temperature of 1.43 Celsius above pre-industrial times.
However, one year with a temperature of 1.5 Celsius warming isn't enough for scientists to declare a breach of that limit, said Carlo Buontempo, director of the Copernicus Climate Change Service, the EU's Earth observation unit.
"In our data, we don't expect to exceed it [1.5 this year] but even if we were, this would only be for one year, while the definition of the Paris Agreement to 1.5 [Celsius] refers to an average number of years," he told DW.
What happens if we cross the 1.5 Celsius threshold?
A World Meteorological Organization report predicts global temperatures will hit new highs in the next five years, and the UN says earth is likely to cross the critical 1.5 Celsius threshold for global warming within the next decade.
"The real discussion is: Are we able to go below 1.5. again later in the century?" C3S director Buontempo added. "We have the tools to make 1.5 possible but that means very, very dramatically [cutting] the amount of greenhouse gas emissions."
Crossing the threshold would not mean immediate disaster for everyone, said Sergey Paltsev, deputy director of MIT's Joint Program on the Science and Policy of Global Change.
"The science does not tell us that if, for example, the temperature increase is 1.51 degrees Celsius, that it would definitely be the end of the world," he explained.
What it does mean, scientists warn, is that storms, heat waves and droughts will be more extreme. And that has far-reaching impacts.
Storms and floods pose a threat to people's homes and state infrastructure, while droughts threaten drinking water supply, as well as food production, causing prices to spike. Heat waves pose a danger to human health, especially among the elderly and vulnerable.
Would the impacts be the same everywhere?
No. Though developing nations contribute least to global emissions, they suffer most from the negative impacts of climate change. Pakistan, for example, is responsible for less than 1% of the world's carbon footprint yet is one of the countries most vulnerable to climate change.
Muhammad Mumtaz, an assistant professor at the Fatima Jinnah Women University in Pakistan whose research focuses on adapting to a changing climate, says the third of the country's population living in urban areas is really feeling the heat.
"More than 40 degrees has been recorded in various cities around Pakistan and one of them has gone up to 51 degrees. So it's very dangerous," said Mumtaz.
Archibong Akpan, a Nigeria-based climate policy expert at the United Nations Framework Convention on Climate Change (UNFCC), points to heat waves and cyclones, coupled with high levels of poverty, as evidence for how global heating is hitting Africa's food production.
"Climate change is already affecting food supplies and crops," he said, adding that an acceleration of existing impacts "will ravage a whole lot of livelihoods."
What can be done about it?
Scientists agree that we can slow the rate of global warming by no longer burning fossil fuels. But even if all human emissions were to stop today, Earth's temperature would continue to rise for several decades, meaning climate change will continue to affect future generations.
So, adapting to the changes in weather in ways that allow people to still meet their basic needs is crucial.
How can we adapt to hotter temperatures?
Many countries, regions and cities have been working on adaptation measures for a long time. In the Netherlands, for example, where more than 50% of land lies below sea level, several cities are being transformed to cope with potential flooding. The adaptation model includes sustainable housing, flexible spaces, and a public transportation system to help evacuate people if necessary.
Residents in Mukuru, the largest informal settlement in the Kenyan capital of Nairobi, created a People's Adaptation Plan that led to an improvement in their water management, roads and sanitation. The plan is also being offered as a model for other settlements in Africa and in developing nations around the world.
Archibong Akpan says many African countries are now taking adaptation seriously but that it remains "on a very low scale," because it is hard to talk about "adaptation when there's no finance."
Developing nations have long been calling on wealthy countries that are disproportionately responsible for warming emissions to compensate them for the impacts of climate change through a designated loss and damage fund. At last year's COP27 climate summit, states agreed to such an initiative in principle. How it will work is yet to be established.
Meanwhile, Muhammad Mumtaz said a Pakistan government initiative urging cities to come up with practical steps to reduce emissions and adapt, has helped open people up to the idea of adaptation, and shows that the population needs education as well as finance.
"They've seen the flooding, they've seen the heat waves, they've seen the drought, so based on this, they believe climate change is happening," he said, adding that what people now need is targeted education in their own language.
"People who have knowledge are willing to adapt and to promote doing things differently," said Mumtaz.
Jeannette Cwienk contributed to this report.
Edited by: Tamsin Walker