Scott Pruitt insincerely asked what's Earth's ideal temperature. Scientists answer | Dana Nuccitelli

In short, from a practical standpoint, as little additional warming as possible

In an interview with Reuters last week, Trump’s EPA administrator Scott Pruitt said,

The climate is changing. That’s not the debate. The debate is how do we know what the ideal surface temperature is in 2100?

Pruitt of course is trying to have a strawman debate, distracting from the fact that not a certain temperature as such is better or worse, but that a change from what we are adapted to is a problem, especially a very rapid change - in either direction, cooling or warming, this causes big disruption.

We should not stray too far away from what we and the currently existing ecosystems have evolved for. That is the optimum, simply because it is what we’re highly adapted to, and any major change is going to be very painful.

There is no one perfect temperature for the earth, but there is for us humans, and that’s the temperature we’ve had over the last few thousands of years when we built our civilization, agriculture, economy, and infrastructure. Global average temperature over the last few millennia has fluctuated by a few tenths of degrees; today, it’s risen by nearly 1°C and counting.Why do we care? Because we are perfectly adapted to our current conditions. Two-thirds of the world’s largest cities are located within a metre of sea level. What happens when sea level rises a metre or more, as it’s likely to this century? We can’t pick up Shanghai or London or New York and move them. Most of our arable land is already carefully allocated and farmed.

What happens when we can no longer grow the crops we used to, as climate shifts and water becomes more scarce in many subtropical areas? We can’t just take over new land: someone else already owns it. What happens when our water resources diminish or even run out? We can’t take over someone else’s water rights without a war.We care about a changing climate because it exacerbates the risks we face today, and threatens the resources we depend on for our future.

There are some absolute temperature thresholds that are important when it comes to agriculture (particularly in tropics), coral bleaching, infectious disease, and heat stress (e.g. the tropics becoming essentially unlivable).

So while it’s difficult to say what the “ideal temperature” is, there clearly is a range of temperatures suitable for human civilization and there is a real danger that we’re pushing toward the upper limit of that range.

Of course, there is no ‘ideal’ temperature so we should not accept that framing. We have strong scientific evidence that anything above 2°C is likely to be deeply problematic. Even the roughly 1°C change we have already seen is having measurable, adverse effects.