Asked to name the biggest thing that’s happened over Resurgence’s 45-year career, I think I’d have to say the melt of the Arctic. When this magazine began publishing, there was 40% more summer sea ice in the Arctic. Viewed from space, in those first pictures from the Apollo spacecraft, the planet looked very different than it does now. In recent summers both the north-west and the north-east passages have opened, allowing sailors to circumnavigate the Arctic through waters that no one thought, even a decade ago, humans would ever navigate.

Or maybe I would pick the rapid acidification of the planet’s seas – they’re 30% more acid than they were in 1966. Which means that the small creatures at the base of the marine food chain are having more trouble forming their shells, and that coral reefs – already stressed by warming waters – have a new trauma to deal with.

Another possibility: the Earth’s atmosphere is about 4% moister than it was 45 years ago, simply because warm air holds more water vapour than cold. This loads the dice for deluge, downpour, flood – it’s not surprising that we’re seeing record rainfall and unprecedented floods. Nor, since that water has to come from somewhere, should increasing drought and desertification come as much of a shock.

Here’s what I’m trying to say: when Resurgence began its run, we were still in the Holocene. Humans had altered much of the planet’s natural environment. We had dirty rivers and dirty air, spreading toxins and endangered species. But the basic operating system of the planet was running pretty much the same as it had for the 10,000 years of human civilisation.

Sometime in the intervening decades we moved out of that comfortable and remarkably stable world, and began the transition to What Comes Next?

Any date would be arbitrary, but if you wanted to pick one, you could say 1988. That was the year NASA scientist Jim Hansen warned the US Congress that global warming was indeed real – and it was the year that we passed the benchmark of 350 parts per million (ppm) of CO2 in the atmosphere. At the time we didn’t know it was a benchmark – it was 20 years later that scientists, again led by Hansen, declared that 350ppm was the absolute upper limit if we wanted a planet “similar to the one on which civilisation developed and to which life is adapted”. If we wanted, in other words, that older world we were born onto.

I could list at some length the various woes this new world is already causing: we see rising sea levels displacing farmers across the deltas of Bangladesh, and Aedes aegypti expanding its boundary and spreading dengue fever like wildfire. Speaking of wildfire, we see record amounts, in part because of more heat and drought, and in part because insects (once kept in check by cold weather) are now spreading.

We see millions still homeless from last year’s flood in Pakistan, and billions struggling to pay for food because a string of crop failures that began with last summer’s Russian drought have increased grain prices by 70–80%.

And I could list at even greater length the woes we expect as the century grinds on. After all, we’ve only raised the temperature about a degree so far, and the climatologists tell us to expect four or five unless we stop burning coal and oil and gas much faster than any government currently plans. Temperatures like that will guarantee the melt of Greenland; according to the agronomists they will cut grain harvests by a third or more; they’ll make current shortages of water seem barely worth mentioning.

But for the moment don’t think about consequences, current or future. Just think about the enormity of what we’ve managed to do: we’ve altered the most basic operations of the one planet we’ve got.

The air is profoundly different, the heat balance with our sun profoundly altered. It’s by far the biggest thing humans have ever done or ever contemplated doing, and were some alien watching from a great distance she’d be scratching her head-like appendage. It’s our head-like appendage that’s responsible, of course. That big brain turned out to be incredibly clever, and its cleverest trick was to figure out that buried carbon could make life easy. Everything that we know around us – the whole modern world – derives from that discovery. Much of it is good. But now we’re threatening to take down the good, and much else with it.

So here’s the question for the next 45 years of Resurgence: can the big brain bail us out?

It’s already provided us with the warnings we need, warnings that would not have been available at any other moment in human history. The scientific method, one of the greatest achievements of our civilisation, has produced a robust consensus on this difficult problem in chemistry and physics: since the mid-1990s the Intergovernmental Panel on Climate Change (IPCC) has said in unmistakably plain language that there is no doubt: we have to cut carbon, and quickly.

But so far that warning has had little or no effect: our national governments, with a few noble exceptions, have paid scant attention; our attempts at global governance have been pathetic failures. The chief reason, I think, is the remarkable power of the fossil-fuel industry to stifle change; for 200 years it has grown bigger and richer and more than able to deal with the threat that science now poses to its reign. It literally makes us stupid: earlier this winter the US House of Representatives, by a 60-vote margin, defeated a resolution that merely stated that global warming was real. Exxon has promoted its own version of physics and chemistry, and they’ve managed to fool a good many too.

The real question, then, is going to be: is there a big enough heart connected to that big brain? Will we be able to heed not only the warnings of science but those of our conscience? Will we, gazing out at the growing array of ‘natural disasters’, figure out that we’ve got to make change? Change in our personal lives, yes, but even more change in our political arrangements, since that’s the only chance that actual physics and chemistry really give us for meeting the deadlines they’ve set.

I think the answer is yes – a tentative and uncertain yes, but one based on just enough real-world data to give me hope.

Three years ago we founded, the first big global grassroots climate campaign. Rooted in science but expressed in imagination, it has grown to pretty mammoth size. Our first two big global days of action, in the waning months of 2009 and 2010, were what CNN called “the most widespread days of political action in the planet’s history”, with nearly 15,000 demonstrations in every country on Earth but North Korea.

And the good news is we didn’t really ‘organise’ it – our tiny staff worked feverishly, but ultimately it was like a potluck supper. People in every corner of the Earth heard the call and did the work, figuring out what would work in their place. (An underwater demonstration on the dying coral reefs of the Maldives; a giant image of King Canute, composed of thousands of volunteers, trying to hold the sea back on the Brighton seashore!)

It’s not enough yet to beat the fossil-fuel industry – our bodies don’t yet add up to their money. But we’re growing constantly (the next big chance to join us: 24 September 2011, a day we’re calling Moving Planet). And – sad, but true – the natural world is going to continue to give us openings to make the case more strongly. Sooner or later our leaders will listen – and we’re committed to making it sooner.

We’re not going to stop global warming: it’s already warmed and it will warm some more. Those Apollo images of our planet are forever sepia-toned. But we’ve got a tiny window left to save something of the Earth we were born onto – its beauty, its bounty, its safety.

That’s our task, and the next 45 years will tell the tale.

Bill McKibben wrote the first book for a general audience on climate change, The End of Nature, in 1989. He is a scholar in residence at Middlebury College in Vermont.