Global warming is a big issue. If we’re honest and we look carefully at the data, it’s beyond question that the atmosphere of our planet is warming. It’s also beyond any honest question that the preponderance of the evidence is that human behavior is the primary cause. It’s not impossible that we’re wrong – but when we look at the real evidence, it’s overwhelming.

Of course, this doesn’t stop people from being idiots.

But what I’m going to focus on here isn’t exactly the usual idiots. See, here in the US, we’re in the middle of a dramatic heat wave. All over the country, we’ve been breaking heat daily temperature records. As I write this, it’s 98 degrees outside here in NY, and we’re expecting another couple of degrees. Out in the west, there are gigantic wildfires, cause by poor snowfall last winter, poor rainfall this spring, and record heat to dry everything out. So: is this global warming?

We’re seeing lots and lots of people saying yes. Or worse, saying that it is, because of the heat wave, while pretending that they’re not really saying that it is. For one, among all-too-many examples, you can look at Bad Astronomy here. Not to rag too much on Phil though, because hes just one among about two dozen different example of this that I’ve seen in the last 3 days.

Weather 10 or twenty degree above normal isn’t global warming. A heat wave, even a massive epic heat wave, isn’t proof that global warming is real, any more than an epic cold wave or blizzard is evidence that global warming is fake.

I’m sure you’ve heard many people say weather is not climate. But for human beings, it’s really hard to understand just what that really means. Climate is a world-wide long-term average; weather is instantaneous and local. This isn’t just a nitpick: it’s a huge distinction. When we talk about global warming, what we’re talking about is the year-round average temperature changing by one or two degrees. A ten degree variation in local weather doesn’t tell us anything about the worldwide trend.

Global warming is about climate. And part of what that means is that in some places, global warming will probably make the weather colder. Cold weather isn’t evidence against global warming. Most people realize that – which is why we all laugh when gasbags like Rush Limbaugh talk about how a snowstorm “proves” that global warming is a fraud. But at the same time, we look at weather like what we have in the US, and conclude that “Yes, global warming is real”. But we’re making the same mistake.

Global warming is about a surprisingly small change. Over the last hundred years, global warming is a change of about 1 degree celsius in the global average temperature. That’s about 1 1/2 degrees fahrenheit, for us Americans. It seems miniscule, and it’s a tiny fraction of the temperature difference that we’re seeing this summer in the US.

But that tiny difference in climate can cause huge differences in weather. As I mentioned before, it can make local weather either warmer or colder – not just by directly warming the air, but by altering wind and water currents in ways that create dramatic changes.

For example, global warming could, likely, make Europe significantly colder. How? The weather in western Europe is greatly affected by an ocean water current called the atlantic conveyor. The conveyor is a cyclic ocean current, where (driven in part by the jet stream), warm water flows north from the equator in a surface current, cooling as it goes, until it finally sinks and starts to cycle back south in a deep underwater current. This acts as a heat pump, moving energy from the equator north and east to western Europe. This is why Western Europe is significantly warmer than places at the same latitude in Eastern North America.

Global warming could alter the flow of the atlantic conveyor. (We don’t know if it will – but it’s one possibility, which makes a good example of something counter-intuitive.) If the conveyor is slowed, so that it transfers less energy, Europe will get colder. How could the conveyor be slowed? By ice-melt. The conveyor works as a cycle because of the differences in density between warm and cold water: cold water is denser than warm water, so the cold water sinks as it cools. It warms in the tropics, gets pushed north by the jet stream, cools along the way and gradually sinks.

But global warming is melting a lot of artic and glacier ice, which produces freshwater. Freshwater is less dense than saltwater. So the freshwater, when it dilutes the cold water at the northern end of the conveyor, it reduces its density relative to the pure salt-water – and that reduces the tendency of the cold water to sink, which could slow the conveyor.

There are numerous similar phenomena that involve changes in ocean currents and wind due to relatively small temperature variations. El Nino and La Nina, conveyor changes, changes in the moisture-carrying capacity of wind currents to carry – they’re all caused by relatively small changes – changes well with the couple of degrees of variatio that we see occuring.

But we need to be honest and careful. This summer may be incredibly hot, and we had an unsually warm winter before it – but we really shouldn’t try to use that as evidence of global warming. Because if you do, when some colder-than-normal weather occurs somewhere, the cranks and liars that want to convince people that global warming is an elaborate fraud will use that the muddle things – and when they do, it’ll be our fault when people fall for it, because we’ll be the ones who primed them for that argument. As nice, as convenient, as convincing as it might seem to draw a correlation between a specific instance of extreme weather and global warming, we really need to stop doing it.