Two degrees - the temperature rise we need to stay under to avoid catastrophic effects of climate change – is now the catch cry for global warming. Governments and numerous NGOs have eagerly adopted the limit; whether we can meet the target is another matter. But 2C isn’t an easy concept to grasp. So, how can we imagine what 2C means for the world?

First, why is 2C so significant? At any time of the year, for most latitudes at which people live, the difference between overnight lows and daytime high temperatures can be as much as 15C. In summer it is even greater. When the day to day temperatures of weather vary so much, 2C seems insignificant.

What we’re talking about here, though, is the global average temperature, not daily variations - and we’re fast approaching 2C warmer than before the industrial revolution and emissions from fossil fuels intensified. With feedbacks, such as increased water vapour (which is a powerful greenhouse gas), loss of reflective ice surface, and potential methane pulses (a greenhouse gas 20-times more powerful than CO2), 2C could be reached before 2060.

But it’s very difficult for people to imagine what a change in average temperature of 2C means for the planet as a whole, because of that daily variation.

Flickr/Ron Wiecki

One way is to imagine what the world was like when it was 2C colder than 1750. Fortunately, the earth has already done the experiment for us, 18,000 years ago in the ice ages.

For starters, there was an ice sheet one mile thick (the Laurentide) that extended over the northern half of North America right down to New York.

But these events happened 18,000 years ago, over a timeframe of hundreds of years, as a result of changes in the earth’s orbit and other natural forces.

What should worry us today is that human-forced climate change is happening at 10,000 times the rate of climate change caused by these natural cycles.

The changes we’re seeing now may be happening quickly compared to the past, but still slowly compared to human lifespans. That’s because the climate system is “lethargic”. And here’s the problem, the inertia of climate is very difficult to imagine or visualise.

There a variety of reasons visualising the inertia of climate change is difficult; prime among them being that climate is complicated. There’s a complex interplay between climate and oceans. For instance, 90% of warming is going into the oceans - at the rate of four Hiroshima bombs per second. But even with all that heat going in the oceans, they will still take a great many decades to warm.

The other complexity is the relationship with ice and climate. Ice may be melting at the poles, but it may be having surprising effects in the Northern hemisphere, like more cold fronts.

And as the atmosphere warms it can hold more water vapour, which can lead to increased rainfall and even snow at higher latitudes.

So we need an image that captures the complexity of the climate system as it gradually approaches 2C, and that climate won’t always appear to warm in a straightforward manner. One way is to think of the planet as a kind of open icebox which has been neglected at a party. The ice inside reacts to the warmer climate outside and melts. As it melts, the resultant water remains cold until all the ice has melted. Eventually the icebox fails to prevent the drinks from heating up.

According to climate scientist James Hansen, we are never going to see another ice age, ever. When the ice melts it will not be replaced. Ice extent will fluctuate from year to year, and some climate change deniers will selectively point to recovery years, but there is only a downward escalator. Which means, unless dangerous climate change is addressed, for some of us today, and many more tomorrow, the party will definitely be over.

This article follows Four Hiroshima bombs a second: how we imagine climate change. Read it here.