Climate science: the core ideas

What you'll learn: how weather differs from climate, and why a single cold winter or hot summer tells you very little about whether the climate is changing.

A question at the harbour

Ines is seventeen and lives in a small fishing town on the coast. Her grandfather Tomás fished out of its harbour for forty years. When a school project asked her to investigate a big question, she picked the one he keeps asking at Sunday lunch: "People say the climate is changing. But the weather has always changed. Last winter was freezing. So how would anyone know?"

It is a fair question, and it is where every course on climate science should start. Over the next eleven lessons we will follow Ines as she fills a notebook with evidence, one idea at a time. Her first discovery is that Tomás has blended two different things together: weather and climate.

Weather is what you get today

Weather is the state of the atmosphere at a particular place and time: the temperature this afternoon, whether it rains tomorrow, how hard the wind blows tonight. Weather changes by the hour and swings a lot from one day to the next. A forecast tries to predict it a few days ahead, and beyond roughly ten days the details become guesswork, because small differences in today's atmosphere grow into large differences later.

Climate is the long-run pattern of weather in a place: the averages, the usual range, and how often extremes happen. Scientists usually describe climate using a period of about 30 years. That is long enough for the random ups and downs of individual years to average out.

A common way to put it: weather is your mood, climate is your personality. Your mood on a given morning can be grumpy or cheerful for all sorts of reasons. Your personality is what people would describe after knowing you for years. One grumpy morning does not change your personality, and one cold winter does not cancel a warming climate. But if you were slightly more irritable on most days for decades, your friends would notice the shift, even though plenty of your days were still cheerful.

Why one cold winter is not evidence either way

Ines looks at the records from the town's weather station, which her geography teacher helped her find. For this lesson, imagine the winter (December to February) averages look like this:

The figures are illustrative, but the pattern is the typical one. Last winter really was cold, well below the recent average. Yet it was still warmer than the coldest winters of earlier decades, and one season sits inside the normal spread of year-to-year variation. The useful comparison is between the two 30-year averages, and that shows a rise of about 0.7 °C.

So Ines writes her first rule in the notebook: to see climate, compare long averages, not single seasons.

Natural variability is real

Tomás is right that weather has always changed, and climate has too. Several natural processes push temperatures up and down from year to year or over longer stretches:

1. El Niño and La Niña. A see-saw of warm and cool water in the tropical Pacific that nudges global average temperature up or down by one or two tenths of a degree for a year or so.
2. Volcanic eruptions. Large eruptions throw reflective particles into the high atmosphere and can cool the planet slightly for one to three years.
3. The Sun. Its output rises and falls over an 11-year cycle, but by a very small amount.
4. Slow orbital changes. Over tens of thousands of years, small wobbles in Earth's orbit helped pace the ice ages.

Climate scientists do not ignore these. They measure them and ask whether they can explain the changes we see. Much of this course is about how that question is answered.

What "climate change" means in practice

When scientists talk about climate change today, they mean a shift in those long-run averages and ranges that persists for decades or longer. The main change being studied is global warming: a rise in Earth's average surface temperature since the late 1800s. Bodies like the Intergovernmental Panel on Climate Change (IPCC), which assesses the published research every several years, put the warming at roughly 1.1 to 1.3 °C above pre-industrial levels by the early 2020s, depending on the exact years and dataset used. These figures are updated as new data arrive, so you will see slightly different numbers in different reports.

A degree or so may sound small. But it is a global average over land and sea, day and night, all seasons. For comparison, the difference between the depths of the last ice age and today was roughly 5 to 7 °C of global average temperature. Small changes in a global average can mean large changes in local weather.

Global versus local

One more distinction goes in Ines's notebook. Her town is one point on the map. Local temperatures are noisier than the global average, because local weather depends on wind direction, sea temperature nearby and many other things. Some places have warmed faster than the global average (the Arctic, most land areas), some more slowly (parts of the Southern Ocean). A single town's record is a clue, not a verdict. That is why later lessons look at how thousands of records from around the world are combined.

Recap

• Weather is the atmosphere's state at a moment; climate is the long-run pattern, usually judged over about 30 years.
• A single cold winter or hot summer cannot confirm or refute a change in climate; compare long averages.
• Natural variability (El Niño, volcanoes, the Sun, orbital cycles) is real and is measured, not ignored.
• Global warming is estimated at roughly 1.1 to 1.3 °C since pre-industrial times by the early 2020s, and the figures keep being updated.
• Local records are noisier than the global average, so they are clues rather than proof.

What you'll learn: how Earth's temperature is set by a balance between sunlight coming in and heat going out, and how greenhouse gases shift that balance.

From averages to causes

Ines now knows to compare long averages, and the averages say her town and the planet have warmed. Tomás's next question is the obvious one: "Fine, it's warmer. But why?" To answer it, Ines needs the single most important idea in climate science: Earth's energy balance.

Energy in, energy out

Earth gets almost all of its energy from the Sun. Averaged over the whole planet, day and night, the top of the atmosphere receives about 340 watts of sunlight per square metre. About 30 percent of that is reflected straight back to space by clouds, ice, snow, bright deserts and tiny particles in the air. The remaining 70 percent or so is absorbed, warming the land, oceans and air.

A warm object gives off energy as infrared radiation, which our eyes cannot see but a thermal camera can. The warmer something is, the more infrared it gives off. Earth does this constantly. Its temperature settles where the infrared going out matches the sunlight being absorbed.

Think of a bath with the tap running and the plug half out. Water flows in from the tap (sunlight) and drains out of the plughole (infrared heat leaving to space). The water level (temperature) rises until the outflow matches the inflow, because a fuller bath pushes water out of the drain faster. If you partly block the drain, the level rises until the extra pressure forces water out as fast as it comes in again. The level then holds steady, but higher than before.

The greenhouse effect

If you calculate the temperature Earth would need to radiate away all its absorbed sunlight directly to space, you get about minus 18 °C. Yet the average surface temperature is about plus 15 °C. The gap of roughly 33 °C is the natural greenhouse effect, and without it Earth would be largely frozen.

Here is how it works. Sunlight is mostly visible light, which passes through the air easily. The infrared that the warm surface sends back up is different: certain gases absorb it. These greenhouse gases then give off infrared of their own in all directions, including back down towards the surface. The heat still escapes to space in the end, but from higher, colder layers of the atmosphere, which radiate less efficiently. To push out enough energy, the whole system has to be warmer.

In bath terms, greenhouse gases partly block the drain.

The surprise for Ines is that the gases making up 99 percent of the air do almost nothing to infrared. Molecules with only two identical atoms, like N₂ and O₂, cannot absorb it in this way. A trace gas like CO₂ can matter enormously because it is one of the few that can.

What changed

The physics of CO₂ absorbing infrared was shown in laboratories in the 1850s and 1860s (work by Eunice Foote and John Tyndall), and in 1896 Svante Arrhenius estimated how much warming more CO₂ would cause. None of this is new science.

What is new is the amount. Before the industrial era, CO₂ was about 280 parts per million (ppm) of the air. It is now above 420 ppm, an increase of roughly 50 percent, and it rises by roughly 2 to 3 ppm each year. Methane has more than doubled. Adding these gases blocks the drain a little more. For a while, Earth absorbs more energy than it sends out, and it warms until the balance is restored at a higher temperature.

Scientists measure this push in watts per square metre and call it radiative forcing. Doubling CO₂ from pre-industrial levels would add a forcing of about 3.7 watts per square metre. That sounds tiny next to 340, but over the whole planet, every second, it is a vast amount of extra energy.

Ruling out the Sun

Could the Sun be the cause instead? Ines checks. Satellites have measured the Sun's output directly since the late 1970s, and it has not risen over that time; if anything it dipped slightly. There is also a fingerprint. If the Sun were warming Earth, the whole atmosphere would warm from top to bottom. Instead, the lower atmosphere has warmed while the stratosphere, high above, has cooled. That is what you expect when greenhouse gases trap heat lower down, so less reaches the upper layers.

Ines writes in her notebook: Earth's temperature is a balance. We have added gases that partly block the heat's way out. The Sun has not got brighter.

Recap

• Earth's temperature settles where absorbed sunlight equals infrared heat radiated to space.
• About 30 percent of incoming sunlight is reflected; the rest is absorbed.
• Greenhouse gases absorb outgoing infrared and re-emit it, so heat escapes from higher, colder layers and the surface must be warmer; naturally this adds about 33 °C.
• Nitrogen and oxygen barely absorb infrared; CO₂, methane and water vapour do.
• CO₂ has risen from about 280 ppm to above 420 ppm; the Sun's output has not risen since satellite measurements began, and the cooling stratosphere points to greenhouse gases.