Changes During The Industrial Age

Most of the 1°C temperature change since the start of the industrial revolution has occurred in the last six decades (Figure 1.1). The warming is evident in all independently monitored timeseries of global temperature. The general warming trend has been overprinted by variability on a lot of different timescales, largely because of internal (re-) distributions of heat within the atmosphere- ocean system. The world ocean, with an average depth of 3700 m, has more than 1000 times the heat capacity of the atmosphere. Even just the upper 700 m that are in effective exchange with the atmosphere have 200 times the heat capacity of the atmosphere. As a result, even a tiny fraction of a degree centigrade change in just the upper ocean represents an enormous amount of heat. This means two things: first, atmospheric temperature can be substantially affected by almost undetectable changes in the ocean; and second, ocean heat gain calculation requires very precise temperature measurements. Precise measurement series for the ocean only exist since about 1960. Let’s have a look at what atmospheric and oceanic heat gains tell us about the Earth’s energy balance since the industrial revolution. The roughly 1°C rise of Earth’s surface temperature during the indus-trial age, with more than two- thirds of it since about 1960, represents the “realized” response to forcing. Using standard values for global climate sensitivity to radiative forcing, we can determine that this 1° C warming corresponds to a component of climate forcing of roughly 1.1 to 1.3 W/m2. In contrast, the ocean is such a vast reservoir to heat up that it has not yet realized its full warming—ocean warming will therefore continue to develop over many decades to centuries even if we managed to “freeze” all radiative forcing agents at their current levels. Since 1960, the heat content of the upper 2000 m of the ocean has increased by roughly 27 x 1022 joules in about 55 years. This is an enormous number; namely 27 followed by 22 zeroes. For comparison, the most powerful nuclear detonation ever had a yield of about 22 x 1016 joules.