全球变化知识资源中心

Where is the heat? That is the question on the minds of many scientists, and many climate change sceptics. The 'global warming hiatus' — the fact that globally averaged air temperatures have not increased as quickly in the past decade as they have in previous decades^{1, 2} — is a hot topic, so to speak. It even has its own spotlight in Chapter 9 of the Working Group I report of the IPCC 5th Assessment Report³.

Temperatures are going up. This decade is warmer than last decade, which is warmer than the decade before that. This response of global temperatures is expected from physical considerations of increased greenhouse gases in our atmosphere. At issue is the decreased rate of temperature increase. Why the rate has slowed seems mysterious. The radiative imbalance at the top of the atmosphere, which drives global temperature increases, has continued to increase over the past several decades. If our planet's energy were in balance, the net shortwave energy coming in from the Sun would be equal to the longwave energy emitted by the Earth, which in turn depends on the temperature of the planet. Currently, Earth's energy imbalance is approximately 0.6 W m⁻² (ref. 4).

The hiatus refers to the fact that even though an energy imbalance persists, the surface temperatures are not increasing as fast as they had been in the previous two decades. Some have pointed to the fact that energy entering the Earth's climate system may have decreased recently due to reductions in solar output⁵, decreases in water vapour in the upper atmosphere⁶ and eruptions from several small volcanoes. However, estimates suggest that these factors could reduce that imbalance at most by half over the last decade. So where is that extra heat going?

Part of the answer is that surface temperatures are only one aspect of heat and energy in our climate system. Energy from the Sun is absorbed at the surface and heats it, but it can also be distributed within the climate system. The majority of the Sun's energy lands in the tropics and then, through ocean and atmosphere circulations, is redistributed to higher latitudes. However, the ocean is not a passive bathtub; its circulation plays a critical role.

Because the ocean is heated from above, the upper part of the ocean is relatively warm, where wind mixing creates fairly uniform conditions to some depth, below which the temperature changes rapidly into the cold, deep, abyssal ocean. The true mixed layer is typically deeper where winds are stronger and in the absence of upwelling. Upwelling in the ocean is caused by the divergence of currents due to both wind divergence and the Earth's rotation. Notable upwelling regions are along the eastern boundary of the ocean basins, and in eastern equatorial oceans, particularly the Pacific Ocean. In these upwelling regions, the heating of the mixed layer by the Sun is offset by the flux of cold water being brought from depth.

The ocean also has downwelling regions. In particular, there are areas of deep convection in the sub-polar Atlantic Ocean and in the Southern Ocean near Antarctica. Here, the water is very cold after losing its heat to the atmosphere and very salty due to salt rejection during ice formation. It is denser than the water beneath it, and it sinks. These areas provide a flux of cold water from the surface to the depths of the ocean and are the driving branch of the global thermohaline circulation. There are several meridional (north–south) overturning circulations that extend deep into the ocean⁷, which are important contributors to ocean heat transport and to the ocean structure. The description above represents a first-order picture of that. It is also a description of the mean state of the climate system. In order to discuss things like a global warming hiatus, or a global warming acceleration, one must consider the role of variability in the climate system.

Known modes of variability in the climate system do influence the exchange of heat between ocean and atmosphere and the distribution of heat within the ocean.

El Niño/Southern Oscillation (ENSO) is a coupled ocean–atmosphere phenomenon of the tropical Pacific, which in the El Niño (or warm) phase results in a warming of eastern equatorial Pacific Ocean surface temperatures with a frequency of about 3–7 years. ENSO is the greatest contributor of natural variability to global temperature changes⁸. The largest El Niño event of the twentieth century was experienced in 1997–98. At that time, 1998 was the warmest year on record. Since then we have experienced several strong La Niña events, which, as opposed to El Niño, manifest as colder than normal sea surface temperatures in the eastern equatorial Pacific and lower global temperatures. The fluctuations between El Niño and La Niña events, and the accompanying changes in the subsurface ocean and overlying atmosphere, embody the ENSO phenomenon⁹. Once the ENSO signature is removed from the global mean temperatures, the residual time series is nearly linear¹.

La Niña events store additional heat in the upper ocean. The increased strength of the trade winds due to the colder eastern Pacific increases the east–west temperature difference, and pushes more of the warmed surface waters westward. That warm water piles up in the west, pushing the volume of warm upper ocean water deeper than usual and thus storing heat below the surface there. Meanwhile, the strong trade winds also lead to stronger upwelling in the east, which brings more cold deep water to the surface. Thus, the same heat flowing from the atmosphere into that colder surface water leads to lower surface temperature increases. During La Niña events there is a net heat uptake by the ocean. However, most of that increased heat storage occurs in the upper 300 m of the tropical ocean, and a corresponding hiatus in the rate of temperature increases is observed in the upper ocean as well¹⁰ (Fig. 1).

The time series show monthly anomalies smoothed with a 12-month running mean with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical coloured bars indicate a 2-year interval following the volcanic eruptions with a 6-month lead (owing to the 12-month running mean), and the 1997–98 El Niño event, again with 6 months on either side. At lower right is the linear slope for a set of global heating rates¹⁰.

The Pacific Decadal Oscillation (PDO)¹¹ shares many similarities with ENSO (Fig. 2). The negative phase exhibits cooler than normal sea surface temperatures in the equatorial Pacific and warmer than normal temperature in the mid-latitude Pacific. Many researchers believe that ENSO's impact on the large-scale atmospheric circulation drives the PDO¹², although other oceanic mechanisms contribute¹³. One of the obvious distinctions between ENSO and PDO is the timescale; PDO phases last 10–40 years, though with considerable year-to-year noise. Another distinction is that the magnitude of temperature anomalies associated with PDO are greater in the mid-latitudes than near the equator, which is the opposite of that for ENSO. Although the equatorial temperature signature is weaker for PDO, it has a greater meridional extent. The associated wind field shows a broad swath of increased trade winds and a speed-up of the subtropical cells — the Pacific shallow overturning circulation that connects the subtropical to the equatorial region. This is thought to play an important role in the PDO based on observations¹⁴ and modelling results¹⁵. Again, this can increase heat storage in the central and western Pacific, perhaps to greater depths (that is, below 300 m) than resulting from La Niña events.

The principal component time series, given below in normalized units, is regressed on global sea surface temperatures to give the map above. The black curve is a 5-year running average. Obtained with permission from Univ. Washington JISAO²⁰.

The Atlantic Multi-decadal Oscillation (AMO) is indexed by the average sea surface temperature anomalies over the North Atlantic Ocean. Its timescale is longer than that of the PDO, and seemingly less noisy. Variations in the strength of the Atlantic meridional overturning circulation, part of the global thermohaline circulation, are thought to be the main driver of the AMO¹⁶. A weakened overturning circulation means less formation of cold deep water, which would result in a relative warming through to the depths of the ocean.

A weakened overturning circulation would also draw less warm water polewards, whereas the North Atlantic has been warm since the mid-1990s. This is not conclusive proof that overturning circulation has not changed as there is considerable difficulty in separating the forced and natural variability over the North Atlantic¹⁷. It could be explained by an increased spin of the ocean gyre through increased trade winds, which would transport warm tropical waters to higher latitudes regardless of changes in sub-polar Atlantic Ocean deep convection.

It is worth noting that the Southern Ocean, the other key source of deep water, has shown a reduction in Antarctic bottom water formation since the 1980s. Observations suggest that this reduction contributed approximately 10% of the ocean heat uptake during that time¹⁸.

Natural variability seems to be capable of accounting for changes in ocean heat uptake of the magnitude experienced. Many recent studies point to the role of PDO in this recent hiatus. What is particularly compelling is that this period has also been one of negative PDO. Further suggestive evidence is that the last period with decade-scale trends in global mean temperature as weak as that experienced since the turn of the century occurred through the 1950s and early 1960s, which was another period dominated by very negative PDO conditions. This shows that hiatus periods are unusual but not unprecedented.

Interestingly, no one really talks about the other side of this situation: global warming acceleration. The mid-1970s through to the mid-1990s was a period of positive PDO and saw an acceleration in warming. If you consider the arguments about the effect of the negative phase on warming, then a positive PDO should result in the opposite. That is, reduce the relative rate of deeper ocean heat increases and instead increase the rate at which surface warming is observed.

Another neglected topic is that negative PDO-like conditions are not inconsistent with climate change. One theory of the tropical Pacific response to increased radiative heating is that the western Pacific warms at a faster rate than the eastern Pacific due to the upwelling in the east. This gradient in heating strengthens the existing temperature gradient along the equatorial Pacific, which strengthens the trade winds, potentially leading to a more La Niña-like mean state¹⁹. There is also the question of possible connections between the Pacific and Atlantic, for which there is little definitive evidence at this point.

Unfortunately, this is not a game. How our climate system responds to human activities has serious implications for society, as does the role of natural variability in our realization of climate change. Although the international community has invested in numerous observational systems — in the oceans, on land and in space — we still lack the long-term and continuous observations, and their synthesis, that are critical to understanding the climate system as a whole. Observing systems must be sustained, and where critical gaps are identified — such as the deep oceans — they should be enhanced. For example, the TOGA/TAO array of buoys in the tropical Pacific, which has been critical to our understanding, monitoring and prediction of ENSO, has fallen into decay and is threatened with extinction. Those buoys have done more than any other single project to mitigate the impacts of climate on society worldwide. Not only should that array be refreshed, it should be expanded into the mid-latitudes of the Pacific Ocean. That could provide an unprecedented view of the processes behind things like the PDO, and perhaps lead to predictions of the next hiatus or acceleration.

The information needed to help manage the risks and opportunities of future climate changes, whether natural or man-made, must be based on solid science. Science starts with good observations and their synthesis, but it cannot stop there. It must serve improved understanding, monitoring, and prediction of interannual to decadal variability and its manifestation against a changing mean climate.

1. Trenberth, K. E. & Fasullo, J. T. Earth's Future 1, 19–32 (2013).
   • Article
2. Tollefson, J. Nature 505, 276–278 (2014).
   • CAS
   • PubMed
   • Article
3. Flato, G. et al. in Climate Change 2013: The Physical Science Basis (eds Stocker, T. et al.) Ch. 9 (IPCC, Cambridge Univ. Press, 2013).
4. Hansen, J., Sato, M., Kharecha, P. & von Schuckmann, K. Atmos. Chem. Phys. 11, 13421–13449 (2011).
   • CAS
   • ADS
   • Article
5. Lockwood, M. & Fröhlich, C. Proc. R. Soc. A 463, 2447–2460 (2007).
   • ISI
   • Article
6. IPCC Climate Change 2007: The Physical Science Basis (eds Solomon S. et al.) (Cambridge Univ. Press, 2007).
7. Talley, L. D., Reid, J. L. & Robbins, P. E. J. Clim. 16, 3213–3226 (2003).
   • Article
8. Trenberth, K. E. J. Geophys. Res. 107, http://doi.org/bvwgbd (2002).
9. Philander, S. G. H. El Niño, La Niña, and the Southern Oscillation 293 (Academic Press, 1990).
10. Balmaseda, M. A., Trenberth, K. E. & Källén, E. Geophys. Res. Lett. 40, 1754–1759 (2013).
    • ADS
    • Article
11. Mantua, N. J., Hare, S. R., Zhang, Y., Wallace, J. M. & Francis, R. C. Bull. Am. Meteorol. Soc. 78, 1069–1079 (1997).
    • ISI
    • Article
12. Alexander, M. A. et al. J. Clim. 15, 2419–2433 (2002).
    • Article
13. Miller, A. J., Cayan, D. R. & White, W. B. J. Clim. 11, 3112–3