Ever since the turn of the twentieth century, global warming has seemed to be at a pause. While carbon dioxide content within the atmosphere has measurably risen, global average temperature has noticeably slowed its rise, contradicting some of our basic climate knowledge about albedo and heat flow. Global average temperatures had risen at an alarming average of 0.12°C per decade between 1951 and 2012, but at a meager 0.05°C per decade between 1998 and 2012. Some have called this unexpected phenomenon the “global warming hiatus,” implying an expected return to the historic trend sometime in the future.
As often happens when a weather event contradicts what is expected – see the polar vortex’s visit to the United States – this phenomenon has drawn the attention of climate change skeptics within politics, the public, and even the scientific world. This instance of skepticism, however, is a little different. Unlike the polar vortex and similar one-off events, scientists were originally unsure as to what caused this hiatus, pointing to several different possibilities. Despite this uncertainty, the underlying message, in the words of the Intergovernmental Panel on Climate Change, was always that global warming is still occurring year-to-year and we might simply have been looking for temperatures in the wrong place. Instead of focusing on just surface temperature, they say, “we should look at the overall picture.”What exactly does that mean though? And what is the current unified hypothesis? Computer models had suggested a theoretical possibility; they suggested that during this hiatus carbon dioxide and other greenhouse gases have rapidly been dissolving our oceans, which can store up to nine times as much heat as Earth’s atmosphere and land combined. This, then, would have occurred in tandem with increased observance of ocean acidification.
Until recently, however, there was little empirical evidence to back up this theory. But that is changing. Data have been collected in a study published in Science to support these computational and theoretical findings. Researchers deployed 3,000 floats to collect data on the salinity and temperature of the ocean in the top 2,000 meters of the world’s oceans. Though they hypothesized that, probably because of its size and location, the Pacific Ocean would act as the largest heat sink, their results were somewhat surprising. It seems instead that the Atlantic and Southern Ocean that have most sequestered heat, while in the Pacific and Indian Ocean, temperatures have risen as they have since 1999. In the Atlantic Ocean, saltier water from the tropics travels up into the poles, and cools and sinks, all while melting polar ice. Because of this melting of freshwater ice, the surface water in the poles is made less dense, which holds back the poleward movement of water from the tropics, putting a control on heat sequestration. This cycle in its entirety sends warmer water to the depths of the Atlantic, although it’s not precisely clear how just yet.
Heat storage in the oceans is not the only possible explanation for this hiatus. Scientists have formulated many different hypotheses as to this global warming hiatus—ranging from increased atmospheric pollution and volcanic ash, to variations in solar activity— and the true cause may be one of these alternatives or many factors working simultaneously. Regardless of the cause, however, the pause in rises in surface temperatures does not mean that the climate isn’t changing, nor should it equate to a pause in our depowering of fossil fuels. Warmer, more acidic seas can still have drastic effects on both climate and ocean life. Additionally, it’s believed that after this temporary hiatus ends, temperatures will rise quicker than ever before—a “raising staircase” beginning at an even higher plateau than in the late 20th century.
The public reaction to this pause also has something to teach us about how we think about climate change. Anthropogenic climate change appears not only as rises in surface temperature, but also as temperature change in our seas—not only in the air around us, but also distant, unfamiliar habitats. It may also be that we need to teach ourselves not to be alarmed only by rises in day-to-day temperatures, but rather to see the problem holistically. What is emitted in one place has an impact on all others, and, by extension, on all life. Climate change is a global problem—even if we don’t always see its effects in our lives—and calls for global solutions.