When I emerged 1km later from the icy water, I’ll never forget looking down at my fingers. They had swollen to the size of sausages. The majority of the human body is water and when water freezes, it expands. The cells in my fingers had frozen, swollen and burst. I had never felt anything so excruciating. My nerve cells were so badly damaged it was four months before I could feel my hands again. I resolved never to do another cold water swim.

Then last year I learned about the melting of the glaciers in the Himalayas and Hindu Kush mountains. As nearly 2 billion people – approximately one in three people on the planet – rely on drinking or irrigation water from these glaciers, I decided it was time to emerge from retirement for another symbolic swim – this time in a glacial lake under the summit of Mount Everest. Considering the potential for instability in regions facing rapidly increasing populations twinned with decreasing natural resources, I returned to training.

What made this swim particularly difficult is that this year, of all years, local authorities mounted a large operation to remove the bodies of climbers who lost their lives on the mountain. So there I was – at 5.3km above sea level, attempting something no one has ever tried before while suffering a vicious case of altitude sickness – and frozen bodies are coming past me as I slowly shuffle higher and higher. To say the least, it is unsettling being reminded of your mortality.

In late May, I reached Lake Pumori, adjacent to the Khumbu Glacier on Everest, and began to prepare mentally to launch myself into a swim. I cranked up P Diddy, glared across the water, fixed my mind on the opposite side of the lake and dived in. At 2C (36F), the water was slightly warmer than at the North Pole but, up in the heavens at the icy tip of the world, breathing is very difficult. Within seconds, I was in trouble – gasping, choking, then vomiting. Then I momentarily went under. The first time I managed to recover easily by pushing myself off the bottom of the shallow lake, but when it happened again I was exhausted and overcome with panic. Some people say that drowning is the most peaceful death. Bollocks.

After it happened a third time, I flapped myself to the edge of the lake. My team mercifully lifted me out, moving my chilled body as quickly down the mountain as they could. That evening, we gathered for a debriefing on what had gone awry and how we could try and fix it. My team gave it to me straight, with team leader Maj-Gen Tim Toyne Sewell deciding on a radical tactical shift.

They talk about SAS standing for speed, aggression and surprise. When I left the regiment, I took that philosophy with me, and it was crucial in my swims in Antarctica, down the Thames, across the Maldives, and across the North Pole.

But my team told me to completely forget the past. Every single thing I had learned in 23 years of swimming I had to forget, he told me, and everything I had learned about speed and aggression as a reservist in the Special Air Service I should ignore. Instead of swimming fast, I had to swim as slowly as possible; instead of the crawl, I had to swim breaststroke; and instead of adopting an aggressive attitude, I needed humility. “You can’t bully Mount Everest,” the Major-General said.

Two days later, on 22 May, we climbed up the mountain as slowly as possible and gathered at the lake, where I lay down on a rock and looked up at the summit of Everest. Humbled, I focused on the glaciers and tried to calm myself in the face of my fear. If I went too slowly, I’d die of cold; too quickly and I’d hyperventilate and drown. I then stood, stepped quietly into the water and swam a measured breaststroke across the expanse towards the spot on the other shore where my team awaited, 1km away. Twenty-three minutes later, I arrived.

I learned two basic lessons on Everest. First, just because something has worked in the past does not mean it will worktoday. Second, different challenges require different mindsets. Now, before I do anything, I ask myself what type of mindset I require to successfully complete the task.

Climate change is the Everest of all problems, the thorniest challenge facing humankind. Just because we have lived in a certain way for so long, and we have consumed the way we have for so long, and populated the earth the way we have for so long, doesn’t mean the decisions we’ve made in the past will work today. All the warning signs are there. When I was born, the world’s population was 3.5 billion. There are now 6.8 billion people on the planet. By 2050, that’s expected to rise to 9.4 billion. What’s more, the Earth’s resources aren’t growing; they’re decreasing – and rapidly.

Last week, I spoke in Oxford at Ted, the “Ideas Worth Spreading” conference, and challenged the audience to consider what radical tactical shift they will take. This may look different for each of us – as world leaders, corporate decision-makers, parents, students or otherwise – as we consider the way we engage with our environment. How do we ensure a healthy, sustainable and peaceful world – a world in which our children have a future?

Moving forward, we must discover our own radical tactical shifts, whether they be in our homes, in our workplaces, in our communities, our countries or our world. Dispense with the assumptions and arrogance of yesterday. Take that step, I said, and commit 100% to doing it. I hope, in some small way, that my swim at the top of the world, which changed me, demonstrates that nothing is impossible. With care and collaboration, it is possible to engage in a discourse of humility and to move beyond dialogue to action.