SEI Senior Scientist Sivan Kartha made these remarks at the UN climate conference in Madrid (COP25) on Dec. 10. He began with a reference to the IPCC’s special report Global Warming of 1.5°C . The rest of his remarks are transcribed below, edited very lightly for clarity.

If we decided not to worry about climate change at all, if we didn’t bother about greenhouse gases whatsoever and we just continued to dig up fossil fuels and burn them and to leak methane from pipelines, if we did nothing at all to stop climate change — the best estimate is that the world, as we release more and more greenhouse gases, would warm maybe 5 degrees or so.

And to many people that just doesn’t seem like that much. But our normal human ways of thinking about temperature and little temperature changes just break down when we try and think on the scale on the whole planet and what it would mean to add enough energy to the planet to warm it by 5 degrees.

scientists at COP25 on stage with Greta Thunberg and Luisa Neubauer

SEI’s Sivan Kartha, second on the right, taking the stage with fellow scientists Youba Sokona and William Moomaw, youth activists Greta Thunberg and Luisa Neubauer, and scientists Ko Barrett and Rachel Cleetus. (Photo: Pablo Blazquez Dominguez / Getty )

Let me put 5 degrees in another light. When the Earth was in the depths of the last Ice Age — when there were sheets of ice 2 kilometers, 3 kilometers, 4 kilometers thick over large portions of the world where there are now major cities with huge populations, and when coastlines were tens of kilometers or hundreds of kilometers from where they are now — the world was only about 5 degrees colder than it is now.

That’s right: the temperature only had to warm by about 5 degrees – just nudge up by 5 degrees – to come out of that frigid state, to melt those several-kilometer-thick ice sheets, to rearrange ecosystems across continents, to drive some species extinct and to allow others to emerge. It, in fact, transformed the surface of the Earth to warm by just 5 degrees, and the Earth then settled into a nice stable climate that, as it turned out, was hospitable to the emergence of human civilization.

So what will happen if we humans cause that to happen again? Another 5 degrees of warming?

What would happen if we caused the world to spin off into whatever the opposite of an ice age is? Or even halfway there? 3 degrees, 4 degrees?

Well, we don’t know. We can’t know, not with any kind of reassuring confidence. We’ve never seen that. We’ve never been there. We don’t know how coral reefs that protect our shores and anchor the ocean’s food webs would respond, especially while oceans are acidifying at the same time. We don’t know how much power and force hurricanes would then pull from the much warmer oceans. We don’t know how the South Asian monsoon would change and what will happen to a billion-plus people whose crops that monsoon waters. We don’t know how fast and furiously wildfires will rage in forests that are suddenly in climate zones where they’re not supposed to be.

Anyone who thinks that we know what it will be like — in fact, that we know well enough to predict the costs of allowing that to happen — and who can decide, oh we can afford that, or decide on behalf of others that they can afford that. That’s not science. That’s looking at an existential risk to humankind and deciding to take a gamble.

That’s madness, and we can’t let that happen.

SEI Senior Scientist Sivan Kartha speaks at a Unite Behind the Science event at COP25, on 10 December 2019. Source: Screenshot from UNFCCC video .

Thank God it doesn’t have to happen. We have the technologies we need, we have the money we need, and we can invest in the alternative sources of energy to meet our need so we can stop digging up those fossil fuels.

But access to those technologies and financial resources is really, really concentrated because we live in such an extraordinarily unequal world. The richest 10% gets more than half of the world’s income each year and causes more than half of the world’s pollution each year. But emissions all around the world — not just among that 10% — needs to get to zero. Even among those whose emissions come from daily activities that merely help them get their basic needs met and merely help them make a modest livelihood.

The only way that we can make emissions go to zero among the whole world is if the world’s more privileged inhabitants — those who have benefited as their societies have developed and gotten prosperous and burned lots and lots of fossil fuels – if they eliminate their emissions and if they also extend the support to the rest of the world, to those who are less privileged and who consume less and who emit less, to enable them to do the same.

Not only is that really the only fair way that it can be done, but it’s also the only practical way. It’s the only way it can get done. It’s the only way that the whole world can be convinced that we’re in this together, we’re fighting a common battle and that we’re helping each other.