9.4: Earth’s Energy Imbalance

Another way to understand the greenhouse effect is to look at the atmosphere, land, and ocean as reservoirs of heat. With increasing amounts of greenhouse gases, the sources of heat—the surface-emitted longwave radiation and its absorption and re-emission by greenhouse gases—exceed the sinks of heat—the escape of longwave radiation into outer space. As a result, the amount of heat in the reservoir increases—like water in a bathtub. In the language of the reservoir model introduced in Chapter 11, the sources (E_in) exceed the sinks (E_out), or

E_in > E_out

(Eq. 12.1)

This imbalance between the incoming radiation from the Sun (i.e., the source) and outgoing radiation from Earth’s surface to space (i.e., the sink) is called Earth’s energy imbalance (e.g., Hansen et al. 2005). Estimates of the energy imbalance indicate that nearly 1 watt of energy per square meter (1 joule per second per square meter of Earth’s surface) remain in the Earth system versus what is leaving via outgoing radiation to space (e.g., Meyssignac et al. 2019; von Schuckman et al. 2020). In short, Earth is accumulating energy. According to the August 2021 report of the Intergovernmental Panel on Climate Change (better known as the IPCC):

• Every decade for the last four decades has been warmer than the one before. 2011–2020 was warmer than 2001–2010, which was warmer than 1991–2000, which was warmer than 1981–1990 (IPCC 2021).
• The last two decades (2001–2020) experienced the most rapid warming, about 1.8°F (1°C) higher than the period 1850–1900 (IPCC 2021).
• The highest temperature rise occurred over land, about 2.86°F (1.59°C); ocean temperatures rose 1.58°F (0.88°C; IPCC 2021).
• The Arctic is warming two to four times faster than the rest of the globe (e.g., AMAP 2021; IPCC 2021; Labe et al. 2020).

The rate of change of human-caused global warming is unlike anything observed in the past 2,000 years (IPCC 2021). Earth’s average temperature has been rising at a rate of about 0.13°F (0.07°C) per decade since 1880. However, since 1981, that rate has more than doubled to 0.32°F (0.18°C) per decade (Lindsey and Dahlman 2023). It’s getting warmer faster.

Global Warming vs. Global Climate Change

In Chapter 1, we noted that scientists have observed a change in Earth’s climate (i.e., average weather conditions) over the past 150 years or so, what is known as global climate change. Global warming is one of the changes scientists have observed. Unfortunately, in the popular media, the terms global climate change and global warming are often used interchangeably. Let’s make it clear: their meanings are distinct.

The increase in Earth’s average temperature—global warming—results primarily from increases in human-generated atmospheric greenhouse gases (though other activities also contribute). The increase in Earth’s energy alters the natural functioning of Earth’s subsystems and causes climate to change. The cryosphere melts. The atmosphere gains heat and moisture. The ocean warms. Global warming is a part of Earth’s changing climate, but many other weather- and climate-related phenomena—melting ice caps, extreme precipitation, droughts, more frequent and stronger hurricanes—also serve as examples of global climate change. The distinction between global warming and global climate change proves important for communicating the multiple changes that are happening as Earth warms. Global warming refers to a (specific) human-caused change. Global climate change refers to the myriad of weather and climate changes that occur as a result of global warming.

One other important note of clarification concerns the oft-remarked-upon (and mistaken) association of local changes with global ones. You may have heard someone comment during a cold-weather spell or snowstorm that global warming can’t be happening because it’s cold or snowing outside. Well, colder weather still happens in a warmer world. Global warming has not eliminated the seasonal cycle. Winter will still be colder than summer.

Temperature records reveal the trend expected in a warming world. The number of days of record heat exceeds the number of days of record cold. In the United States, record high temperatures occur twice as often as low temperature records (Meehl et al. 2009; Associated Press 2019). In Australia, that ratio is 12 to 1 (Lewis and King 2015). When you add it all up—across the entire globe—Earth is getting warmer. And we’re the cause of it.

The Ocean’s Temperature Is Rising

Because warming air temperatures affect us directly, you’d be forgiven if you hadn’t thought about the effects of global warming on the ocean. But in fact, the effects of global warming on the ocean are perhaps more alarming than those on land. That’s because the ocean has absorbed most of the heat that otherwise would have ended up in the atmosphere. A warming world ocean signals profound changes for the ocean and will literally change the world as we know it.

Science writers LuAnn Dahlman and Rebecca Lindsey refer to the ocean as Earth’s “largest solar energy collector” (Lindsey and Dahlman 2020). As early as 1956, famed meteorologist Carl-Gustaf Rossby (1898–1957) postulated that “anomalies in heat probably can be stored and temporarily isolated in the sea” (Rossby 1959). In 2000, Sydney Levitus and colleagues published the first estimates of the amount of heat being absorbed by the upper ocean (to depths less than 9,842 feet, or 3,000 meters; Levitus et al. 2000). Their results confirmed Rossby’s hypothesis: Most heat from global warming has gone into the ocean. Subsequent studies using observations from Argo floats and other newly available data substantiated the results more definitively: The world ocean has absorbed 93 percent of the excess heat associated with Earth’s energy imbalance since 1971 (Levitus et al. 2012; Rhein et al. 2013; Meyssignac et al. 2019; Cheng et al. 2023).

Oceanographers refer to ocean-absorbed energy as ocean heat content, the amount of heat energy stored within the ocean (Cheng et al. 2017). Cheng et al. (2020) estimate that since 1960, ocean heat content across all depths of the world ocean has increased by 370 ± 81 zettajoules (1 zettajoule = 10²¹ joules). Now, I admit that it’s difficult to wrap your head around joules much less zettajoules. But Levitus and colleagues (2012) calculated that if the excess heat in the upper 6,562 feet (2,000 meters) was instantaneously released—240 zettajoules since 1955 using their data—the atmosphere would warm by 65°F (36°C). For all depths of the world ocean—using the estimates of Cheng et al. 2020—that temperature comes close to 100°F (37.7°C). Of course, an instantaneous release of heat from the ocean to the atmosphere is impossible. Nevertheless, this calculation underscores the tremendous amount of heat added to the world ocean as a result of human-caused global warming.

Upper ocean heat content set new records in 2021 and 2022, the warmest in human history (Cheng et al. 2023). Not surprisingly, ocean surface temperatures in different locations also set records. In August 2020, the sea surface temperature at Scripps Pier in La Jolla, California, reached 79.5°F (26.4°C)—11.5°F above average (6.4°C)—beating a record set only two years previously (Anderson 2020). The long-term trend in ocean warming caused by climate change accounts for these temperatures (e.g., Fumo et al. 2020).

To conclude, the ocean stores tremendous quantities of heat. The good news is the presence of an ocean on our planet means that Earth’s temperature has risen more slowly than it might have otherwise. The bad news is that even if we were to somehow stop greenhouse gas emissions today, it would take centuries for the ocean and atmosphere to return to normal. That’s because the ocean circulates on timescales of centuries to millennia (Abram 2018; Cheng et al. 2020). So heat stored in the ocean will remain with our planet for a long time. Such a realization makes it ever more urgent that we act immediately to reduce greenhouse gas emissions and put Earth on a path toward a more sustainable and habitable future.