Once you've felt true cold, it never leaves you. It stays in your memory, and deep in your bones.
The cumulative impact of freezing water on the human body is brutal. I've swum until I couldn't feel my fingers and feet. I've swum until my tongue froze inside my mouth. I've swum until I could feel my body shutting down.
Cold is not something I enjoy. But as our planet warms, I'm about to get very, very cold.
This week, I start my multi-day swim across the mouth of Greenland's Ilulissat Icefjord. The water will be close to freezing.
It will be the longest, coldest swim of my life. But I have to do it, to show the world just how rapidly our planet is warming.
Greenland to Glasgow
I believe there is no better place in the world than Ilulissat to show the dramatic impact of the climate crisis, which is why I have chosen to swim here in the lead-up to the climate-change negotiations at COP26.
Ilulissat, on the west coast of Greenland, is the world's fastest-moving glacier. It drains around 30 cubic kilometres of ice into the Davis Strait every year.
Scientists keep a close eye on this glacier to understand how it is being affected by global warming. They've found that warming air and ocean temperatures are causing the glacier to melt at an accelerating scale and pace – an average of 30 metres per day, and even faster in summer.
If the entire Greenland Ice Sheet were to melt, it would lead to a global sea-level rise of over seven metres. It will only take a one-metre rise to drown major cities like London, Tokyo and New York.
With this swim, I will be sending an urgent message to world leaders, before they gather in Glasgow for COP26 in November, where they will make key decisions that will affect us all.
Our planet is in peril, and we need urgent action right now, before it is too late.
No one has ever attempted a multi-day swim in such cold water before. My swim across the mouth of the Ilulissat Glacier will probably take me two weeks. It will be exponentially more challenging than anything I have ever done.
The direct distance across the mouth of the Icefjord is 10 kilometres, but my swim may be considerably longer. My route will be complicated; we will need to navigate around icebergs and sharp brash ice.
No one has tested the effects of swimming, day after day, in water that can drop to minus 1.7°C – and that's before you take the wind chill factor into account.
In Ilulissat I will do this for at least 10 consecutive days, maybe more. Each day I will try to swim for one kilometre, then reheat, recover and start over again the next day.
I don't know how my body will cope.
We rely on ice
Water is my element. But when it gets close to freezing point – which in the case of sea water is minus 1.7°C — it becomes solid and unforgiveable. It's not somewhere you want to be in a pair of swimming trunks.
And yet, I have a deep appreciation and respect for ice. We rely on ice for our survival. It keeps our planet's temperature in a range within which we can live. No ice, no life.
And around 800 million people depend on meltwater from hundreds of thousands of mountain glaciers that are rapidly shrinking.
As our planet heats up, ice melts, sea levels rise, and lives are on the line
Tip of the iceberg
What we are seeing here in Ilulissat is the proverbial tip of the iceberg. The melting we see on the surface is only part of the problem, and only the start of it.
Unlike me, this planet does not have cold deep within its bones. Instead, our Earth has reserves of CO2 and methane gas that, if released, will make everything warmer still.
Getting down to business
I will be taking my message to COP26, not just to world leaders, but to business leaders, investors and consumers. We all have a part to play.
We know that we can influence the decisions companies make, and direct them to create a better, more robust and sustainable world. The bottom line is, we are not going to solve the climate crisis unless we change the way we invest.
I am delighted to have Legal & General Investment Management (LGIM) as my global partner for this expedition. They understand the enormous impact that every investment decision has on our collective future.
They also understand that COP26 is a key moment.
It comes as the signs of a changing planet are all around us – in freak storms, flash floods and runaway fires. It's easy to feel helpless when faced with these forces of nature.
I'm doing this swim to show the world that we are not helpless. We are in control of the decisions we make.
We just need to make sure they are the right ones – and today.
Photo credit: Olle Nordell