[ ♪ Intro ].

If you’ve been outside, like, even one time in your life, you know that the Sun plays a key role in the weather. It’s usually warmer during the day and colder at night, and a cloudy day feels a lot different from a sunny one.

But that’s pretty obvious. Here’s a harder question: How much does the Sun affect Earth’s climate? Like, beyond the fact that it keeps us from being a frozen hunk of space rock?

After all, the Sun goes through all kinds of cycles and changes, so it seems like at least some of that should affect our planet, too. It turns out that it does -- and has for billions of years. But it’s definitely not the only variable.

When scientists talk about the planet’s climate, they’re referring to long-term patterns of weather that can only be seen over long periods of time, like tens, hundreds, or even thousands of years. All kinds of things can influence this, so it makes sense that changes with the Sun, like with its energy output, would, too. After all, that’s mainly where we get our heat.

The thing is, though, some solar changes don’t always do what we think they should. One way we can track the Sun is through the solar constant, which measures roughly the average amount of energy striking every square meter of the Earth. We’ve measured it at about 1.4 kilowatts per meter squared, but studies of other Sun-like stars suggest that number could have changed.

The solar constant isn’t very constant at all. In fact, early in Earth’s life, it was likely 25 or 30% lower than what it is today, thanks to how processes inside the Sun have changed over its lifetime. As a result, basic physics suggests that three billion years ago,.

Earth should’ve been as frozen as Hoth from Star Wars. And some models suggest the Earth would’ve been frozen solid for something like two billion years after its formation. The problem is, that totally didn’t happen according to the fossil record.

There’s also evidence that Mars was warm and wet early on, exactly when the Sun should’ve been dim and cool. Planetary scientists often call this apparent contradiction the faint young Sun paradox. Since it was first realized in the late 1960s, astronomers, geologists, and climate scientists have tried basically every idea under the Sun to explain what happened.

Some scientists think the Sun must be unusual or that Earth’s orbit and rotation had to have been very different. Others argue that maybe clouds or the greenhouse effect work differently than we think. Fifty years later, we still don’t have a good answer for why the Sun didn’t affect Earth as much as it should’ve back then.

But this paradox does make one thing clear: Even if the Sun can change our climate, it’s not the only thing that affects it. Even over shorter and more recent time spans, it isn’t as constant as it might seem, and evidence suggests that those little changes have affected our planet, too -- at least, to a point. For hundreds of years, scientists have tracked a pattern of activity on the Sun called the solar cycle.

Over the course of each 11-year cycle, magnetic forces in the Sun cause big variations in things like cool sunspots and explosive solar flares. Evidence recorded in the rings of trees also hints at longer and more subtle cycles spanning hundreds or even thousands of years. It’s hard to know for sure, but some of these cycles seem to match up with ancient changes in the climate.

For instance, one study tracking solar activity from about 6000 years ago matched up periods when the Sun was calm with wetter environments here on Earth. But it’s not like the Sun directly caused floods or anything, so to know the whole picture, we’d need to understand everything else happening around that time. In general, our star does seem to have played some role in our climate, but there’s also a lot going on down here.

If nothing else, though, one thing we can say for sure is that these long-term solar patterns are definitely not causing the global warming of the last century. At most, the solar cycle results in a change in heat from the Sun of about 0.1%, or nearly 60 times less than what would cause the heating we see today. And, in fact, while the last few decades have experienced the most dramatic warming in millions of years, the Sun has been going through one of its least active periods on record.

There’s only one culprit for the rapid climate changes Earth is experiencing: us troublesome humans. The overwhelming scientific consensus is that the recent warming of the Earth is due primarily to our production of greenhouse gases. There are lots of these, carbon dioxide possibly the most famous, and they’re called that because they contribute to the greenhouse effect, or trapping of heat by Earth’s atmosphere.

That heat first arrives at the Earth as visible light from the Sun, but eventually it gets re-emitted by the surface in the infrared. Then, greenhouse gases block much of that infrared energy from escaping out to space, so it sticks around and warms up our climate. Now, some amount of greenhouse gas is a good thing, because it helps us stay warm.

But too much -- as you might already know -- is a really big problem. At the end of the day, the Sun dominates almost every aspect of life here on Earth. But that doesn’t mean we’re off the hook, either.

We’re a pretty big variable all on our own. Thanks for watching this episode of SciShow Space! If you’d like to learn more about how scientists study the Earth’s climate and predict our future, you can watch our episode all about climate modeling over at the main SciShow channel. [ ♪ Outro ].