Just like your car doesn’t reach top speed the instant you step on the gas, Earth’s temperature doesn’t react instantly to each year’s new record-high carbon dioxide levels. Thanks to the high heat capacity of water and the huge volume of the global oceans, Earth’s surface temperature resists rapid changes. Said another way, some of the excess heat that greenhouse gases force the Earth’s surface to absorb in any given year is hidden for a time by the ocean. This delayed reaction means rising greenhouse gas levels don’t immediately have their full impact on surface temperature. Still, when we step back and look at the big picture, it’s clear the two are tightly connected.
As the graph above shows, both global temperature (colored bars) and atmospheric carbon dioxide (gray line) increased more slowly during the first half of the observational record in the late nineteenth and early twentieth centuries. Atmospheric carbon dioxide levels rose by around 20 parts per million over the 7 decades from 1880–1950, while the temperature increased by an average of 0.04° C per decade.
Over the next 7 decades, however, carbon dioxide climbed nearly 100 ppm—5 times as fast! To put those changes in some historical context, the amount of rise in carbon dioxide levels since the late 1950s would naturally, in the context of past ice ages, have taken somewhere in the range of 5,000 to 20,000 years; we’ve managed to do it in about 60. At the same time, the rate of warming averaged 0.14° C per decade. The rapid rate of temperature rise over such a short period time points to only one thing, and that is the addition of greenhouse gases, primarily carbon dioxide, into the environment.
Within any given decade, however, the temperature bounces around between warm and cool years. The warmest years are usually El Niño years, when the eastern and central tropical Pacific is warmer than average. The coldest years are generally La Niña years. On a longer time scale, warm decades are often associated with strongly positive phases of the Pacific Decadal Oscillation, and cool decades with strongly negative phases.
And while these natural climate patterns—through which the ocean alternately accumulates and releases heat—are the most important cause of short-term variations in global surface temperature, other factors occasionally contribute: volcanic eruptions, solar variability, and smoke and other pollution particles.
Pros and cons of thermal inertia
The global ocean buffers Earth’s temperature from rapid change; that stability has been fundamental to the evolution of complex life on our planet over millions of years. Even with respect to global warming, the ocean’s inertia works in our favor in one way: it provides us with a modest window of time to adapt to and begin to combat climate change before we are forced to confront its full effects on human health, coastal communities, and agriculture.
But there’s a downside to the delayed reaction, too. Like a speeding train, the warming won’t stop the instant we hit the brakes. At whatever point we manage to halt or reverse the trend in greenhouse gases, some additional warming will occur due to the heating imbalance that is already in the pipeline. A recent special report from the Intergovernmental Panel on Climate Change (IPCC) estimates how much we could currently expect based on emissions to date:
“If all anthropogenic emissions…were reduced to zero immediately, any further warming beyond the 1°C already experienced would likely be less than 0.5°C over the next two to three decades (high confidence), and likely less than 0.5°C on a century time scale (medium confidence)…. A warming greater than 1.5°C is therefore not geophysically unavoidable: whether it will occur depends on future rates of emission reductions.”
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