Lewis recently made waves when he broke long-distance swimming records in the icy waters of both the Arctic and Antarctic. In doing so, he became the first person to complete long-distance swims in all five of the world's oceans.

The Arctic swim - in which Lewis covered more than 1km in water of just 3°C, wearing nothing but a pair of Speedos, a cap and goggles, prompted the governor of Svalbard, Norway, to exclaim: "He is a polar bear!"

The motivation
Lewis wants to use his latest challenge to raise awareness of climate change. In the Arctic he saw first-hand the effect that climate change is having on this region's fragile beauty - rapidly retreating glaciers and melting sea-ice threaten its existence and that of Lewis' namesake, the polar bear.

But it's not just the Arctic - the impacts of climate change will be felt globally including in regions such as the Thames and the south-east of England.

By swimming the Thames, Lewis hopes to highlight the need for us all to take action on climate change now.

Climate change, the arctic and polar bears
Nowhere on Earth is the evidence of climate change more stark than in the Arctic.

• Due to increasing CO₂ emissions, the surface temperature of the Arctic has risen by up to 4°C in the last 50 years.
• Satellite data has shown an area of sea ice 5 times the size of the UK has disappeared over the last 4 years.
• Summer sea ice in the Arctic could disappear entirely by 2080.
• These dramatic changes are threatening Arctic people, and species such as the polar bear, walrus and bowhead whale.