Since 1979 we’ve kept watch on the Arctic sea ice pack using satellites. It grows and decays with the seasons of course; more ice in winter/spring, less in summer/fall. But over the decades, we’ve also seen it waste away from year to year.
We get an even better view if we show the average for each year:
Now it’s obviously getting smaller. Not every year, of course — it does so in fits and starts, always fluctuating about — but the long term pattern, the trend, is clear. Deny it, and you are a denier.