Climate Change Evidence & Causes

 

9

Does the rate of warming vary from one decade to another?

Yes. The observed warming rate has varied from year to year, decade to decade, and place to place, as is expected from our understanding of the climate system. These shorterterm variations are mostly due to natural causes, and do not contradict our fundamental understanding that the long-term warming trend is primarily due to human-induced changes in the atmospheric levels of CO2 and other greenhouse gases.

Even as CO2 is rising steadily in the atmosphere, leading to gradual warming of Earth’s surface, many natural factors are modulating this long-term warming. Large volcanic eruptions increase the number of small particles in the stratosphere. These particles reflect sunlight, leading to short-term surface cooling lasting typically two to three years, followed by a slow recovery. Ocean circulation and mixing vary naturally on many time scales, causing variations in sea surface temperatures as well as changes in the rate at which heat is transported to greater depths. For example, the tropical Pacific swings between warm El Niño and cooler La Niña events on timescales of two to seven years. Scientists study many different types of climate variations, such as those on decadal and multi-decadal timescales in the Pacific and North Atlantic Oceans. Each type of variation has its own unique characteristics. These oceanic variations are associated with significant regional and global shifts in temperature and rainfall patterns that are evident in the observations.

Warming from decade to decade can also be affected by human factors such as variations in emissions of greenhouse gases and aerosols (airborne particles that can have both warming and cooling effects) from coal-fired power plants and other pollution sources.

These variations in the temperature trend are clearly evident in the observed temperature record [Figure 4]. Short-term natural climate variations could also affect the long-term human-induced climate change signal and vice-versa, because climate variations on different space and timescales can interact with one another. It is partly for this reason that climate change projections are made using climate models (see infobox, p.20) that can account for many different types of climate variations and their interactions. Reliable inferences about human-induced climate change must be made with a longer view, using records that cover many decades.

 

10

Did the slowdown of warming during the 2000s to early 2010s mean that climate change is no longer happening?

No. After the very warm year 1998 that followed the strong 1997-98 El Niño, the increase in average surface temperature slowed relative to the previous decade of rapid temperature increases. Despite the slower rate of warming, the 2000s were warmer than the 1990s. The limited period of slower warming ended with a dramatic jump to warmer temperatures between 2014 and 2015, with all the years from 2015-2019 warmer than any preceding year in the instrumental record. A short-term slowdown in the warming of Earth’s surface does not invalidate our understanding of long-term changes in global temperature arising from human-induced changes in greenhouse gases.

Decades of slow warming as well as decades of accelerated warming occur naturally in the climate system. Decades that are cold or warm compared to the long-term trend are seen in the observations of the past 150 years and are also captured by climate models. Because the atmosphere stores very little heat, surface temperatures can be rapidly affected by heat uptake elsewhere in the climate system and by changes in external influences on climate (such as particles formed from material lofted high into the atmosphere from volcanic eruptions).

More than 90% of the heat added to the Earth system in recent decades has been absorbed by the oceans and penetrates only slowly into deep water. A faster rate of heat penetration into the deeper ocean will slow the warming seen at the surface and in the atmosphere, but by itself it will not change the long-term warming that will occur from a given amount of CO2. For example, recent studies show that some heat comes out of the ocean into the atmosphere during warm El Niño events, and more heat penetrates to ocean depths in cold La Niñas. Such changes occur repeatedly over timescales of decades and longer. An example is the major El Niño event in 1997–98 when the globally averaged air temperature soared to the highest level in the 20th century as the ocean lost heat to the atmosphere, mainly by evaporation.

Even during the slowdown in the rise of average surface temperature, a longer-term warming trend was still evident (see Figure 4). Over that period, for example, record heatwaves were documented in Europe (summer 2003), in Russia (summer 2010), in the USA (July 2012), and in Australia (January 2013). Each of the last four decades was warmer than any previous decade since widespread thermometer measurements were introduced in the 1850s. The continuing effects of the warming climate are seen in the increasing trends in ocean heat content and sea level, as well as in the continued melting of Arctic sea ice, glaciers and the Greenland ice sheet.