What is global warming, really?

The greenhouse effect, in one diagram

Sunlight passes through the atmosphere fairly easily and warms the ground. The warmed Earth then radiates that energy back outward as heat (infrared radiation). Some of that heat escapes straight to space — but greenhouse gases like carbon dioxide (CO2), methane and water vapour absorb part of it and radiate it back down again, the way a blanket holds your body heat in rather than creating any heat of its own.

That's a genuinely good thing in the right amount — without any greenhouse effect at all, Earth's average surface temperature would sit somewhere around −18°C, far too cold for the world as we know it. The trouble starts when we keep adding far more of these gases than natural processes can absorb, thickening the "blanket" year after year.

Weather vs. climate vs. global warming

These three get mixed up constantly, so here's the difference in one line each:

• Weather is what's happening outside right now — today's rain, today's heat.
• Climate is the long-term pattern of weather in a place, usually averaged over 30 years.
• Global warming is the measured rise in Earth's average surface temperature over decades, caused mainly by human-added greenhouse gases — one driver reshaping climates everywhere, at different speeds and in different ways.

A cold week somewhere doesn't disprove global warming any more than one hot afternoon proves it. It's the decades-long average that matters — and that average keeps climbing.

Why a few hundred parts per million matters so much

"Parts per million" (ppm) sounds tiny, and in one sense it is — CO2 is still a small slice of the atmosphere, which is mostly nitrogen and oxygen. But greenhouse gases are disproportionately powerful at trapping heat for their size. Atmospheric CO2 has risen from about 280 ppm before the Industrial Revolution to roughly 427 ppm in 2025 — about a 50% increase — and each extra molecule adds a little more trapped heat across the entire planet, all the time.