Why Isn’t Every Year the Warmest Year on Record?
This just in: 2022 effectively tied for the fifth warmest year since 1880, when our record starts. Here at NASA, we work with our partners at NOAA to track temperatures across Earth’s entire surface, to keep a global record of how our planet is changing.
Overall, Earth is getting hotter.
The warming comes directly from human activities – specifically, the release of greenhouse gases like carbon dioxide from burning fossil fuels. We started burning fossil fuels in earnest during the Industrial Revolution. Activities like driving cars and operating factories continue to release greenhouse gases into our atmosphere, where they trap heat in the atmosphere.
So…if we’re causing Earth to warm, why isn’t every year the hottest year on record?
As 2022 shows, the current global warming isn’t uniform. Every single year isn’t necessarily warmer than every previous year, but it is generally warmer than most of the preceding years. There’s a warming trend.
Earth is a really complex system, with various climate patterns, solar activity, and events like volcanic eruptions that can tip things slightly warmer or cooler.
While 2021 and 2022 continued a global trend of warming, they were both a little cooler than 2020, largely because of a natural phenomenon known as La Niña.
La Niña is one third of a climate phenomenon called El Niño Southern Oscillation, also known as ENSO, which can have significant effects around the globe. During La Niña years, ocean temperatures in the central and eastern Pacific Ocean cool off slightly. La Niña’s twin, El Niño brings warmer temperatures to the central and eastern Pacific. Neutral years bring ocean temperatures in the region closer to the average.
El Niño and La Niña affect more than ocean temperatures – they can bring changes to rainfall patterns, hurricane frequency, and global average temperature.
We’ve been in a La Niña mode the last three, which has slightly cooled global temperatures. That’s one big reason 2021 and 2022 were cooler than 2020 – which was an El Niño year.
Overall warming is still happening. Current El Niño years are warmer than previous El Niño years, and the same goes for La Niña years. In fact, enough overall warming has occurred that most current La Niña years are warmer than most previous El Niño years. This year was the warmest La Niña year on record.
Our Sun cycles through periods of more and less activity, on a schedule of about every 11 years. Here on Earth, we might receive slightly less energy – heat – from the Sun during quieter periods and slightly more during active periods.
At NASA, we work with NOAA to track the solar cycle. We kicked off a new one – Solar Cycle 25 – after solar minimum in December 2019. Since then, solar activity has been slightly ramping up.
Because we closely track solar activity, we know that over the past several decades, solar activity hasn’t been on the rise, while greenhouse gases have. More importantly, the “fingerprints” we see on the climate, including temperature changes in the upper atmosphere, don’t fit the what we’d expect from solar-caused warming. Rather they look like what we expect from increased greenhouse warming, verifying a prediction made decades ago by NASA.
Throughout history, volcanoes have driven major shifts in Earth’s climate. Large eruptions can release water vapor – a greenhouse gas like carbon dioxide – which traps additional warmth within our atmosphere.
On the flip side, eruptions that loft lots of ash and soot into the atmosphere can temporarily cool the climate slightly, by reflecting some sunlight back into space.
Like solar activity, we can monitor volcanic eruptions and tease out their effect on variations in our global temperature.
At the End of the Day, It’s Us
Our satellites, airborne missions, and measurements from the ground give us a comprehensive picture of what’s happening on Earth every day. We also have computer models that can skillfully recreate Earth’s climate.
By combining the two, we can see what would happen to global temperature if all the changes were caused by natural forces, like volcanic eruptions or ENSO. By looking at the fingerprints each of these climate drivers leave in our models, it’s perfectly clear: The current global warming we’re experiencing is caused by humans.