Periodically, we receive queries asking if Earth is cooling. Although multiple lines of converging scientific evidence show conclusively that our climate is warming, stories sometimes appear in the media calling that into question. New studies are interpreted as contradicting previous research, or data are viewed to be in conflict with established scientific thinking.
Last spring, for example, a number of media outlets and websites reported on a story that looked at data acquired from NASA’s Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP), which estimates changes in global surface temperature. The article discussed a short-term cooling period that showed up in the data in 2017 and 2018 and correctly stated that short-term cooling cycles are “statistical noise compared to the long-term trend.”
Afterward, we received some queries from readers who wanted to know if this finding meant a significant period of global cooling either could be or already was under way.
The answer is no. This story is a great example of why focusing on just a short period of time – say, one, two or even several years — doesn’t tell you what’s really going on with the long-term trends. In fact, it’s likely to be misleading.
So, what’s really important to know about studying global temperature trends, anyway?
Well, to begin with, it’s vital to understand that global surface temperatures are a “noisy” signal, meaning they’re always varying to some degree due to constant interactions between the various components of our complex Earth system (e.g., land, ocean, air, ice). The interplay among these components drive our weather and climate.
For example, Earth’s ocean has a much higher capacity to store heat than our atmosphere does. Thus, even relatively small exchanges of heat between the atmosphere and the ocean can result in significant changes in global surface temperatures. In fact, more than 90 percent of the extra heat from global warming is stored in the ocean. Periodically occurring ocean oscillations, such as El Niño and its cold-water counterpart, La Niña, have significant effects on global weather and can affect global temperatures for a year or two as heat is transferred between the ocean and atmosphere.
This means that understanding global temperature trends requires a long-term perspective. An examination of two famous climate records illustrate this point.
You may be familiar with the Keeling Curve (above), a long-term record of global carbon dioxide concentrations. It’s not a straight line: The curve jiggles up and down every year due to the seasonal cycling of carbon dioxide. But the long-term trend is clearly up, especially in recent decades. As countries around the world rapidly develop and gross domestic products increase, human-produced emissions of carbon dioxide are accelerating.
During fall and winter in the Northern Hemisphere, when trees and plants begin to lose their leaves and decay, carbon dioxide is released in the atmosphere, mixing with emissions from human sources. This, combined with fewer trees and plants removing carbon dioxide from the atmosphere, allows concentrations to climb in winter, reaching a peak by early spring. During spring and summer in the Northern Hemisphere, plants absorb a substantial amount of carbon dioxide through photosynthesis.
Similarly, the above graph of long-term independent global temperature records maintained by NASA, NOAA and the UK’s Climatic Research Unit doesn’t show perfectly straight lines, either. There are ups and downs, and depending on when you start and stop, it’s easy to find numerous periods spanning multiple years where no warming occurred or when global temperatures even decreased. But the long-term trend is clearly up. To learn more about the relationship between carbon dioxide and other greenhouse gases and climate change, visit NASA’s Global Climate change website.
Growing Confidence in Earth Temperature Measurements
Scientists continue to grow increasingly confident that measurements of Earth’s long-term temperature rise in recent decades are accurate. For example, an assessment published earlier this year1 of the agency’s GISTEMP record of global temperatures found that NASA’s estimate is accurate to within less than one-tenth of a degree Fahrenheit in recent decades. They concluded that Earth’s approximately 1 degree Celsius (2 degrees Fahrenheit) global temperature increase since 1880 can’t be explained by any uncertainty or data error. The recent trends were also validated with data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite.
Global Warming Is 'Global'
What’s perhaps most important to remember about global surface temperature fluctuations is that despite short-term ups and downs, the evidence shows that our planet is steadily accumulating heat. Scientists assessing global warming study Earth’s entire heat content, not just what happens in one part of the atmosphere or one component of the Earth system. And what they have found is that the balance of energy in the Earth system is out of whack: Our lower atmosphere is warming, the ocean is accumulating more energy, land surfaces are absorbing energy, and Earth’s ice is melting.
A study by Church et al. (2011) found that since 1970, Earth’s heat content has risen at a rate of 6 x 1021 Joules a year. That’s the equivalent of taking the energy output of about 190,000 nuclear power plants and dumping it into the ocean every year.
Despite short-term decreases in global temperature, the long-term trend shows that Earth continues to warm.
- Lenssen, N., G. Schmidt, J. Hansen, M. Menne,A. Persin,R. Ruedy, and D. Zyss, 2019: Improvements in the GISTEMP uncertainty model. J. Geophys. Res. Atmos., early view, doi:10.1029/2018JD029522.
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