2

The Ocean and the Climate System

2.1. Introduction

Global warming, now well attested on a global scale, remains an important subject of study since its characteristics are difficult to typify, on account of its complexity and the inadequacy of our current knowledge of the state of the ocean. Anticipating how the intrinsic properties that characterize the ocean contribute to shaping the state of the climate and its evolution remains a major challenge. After presenting some findings concerning the changes already observed and methods for studying future projections, we will return to the behavior of the ocean and the way in which it could influence the evolution of the climate by its particular dynamics and rhythm.

2.2. Climate change

The Earth has experienced greatly contrasting climates, from its "snowball" stage, 640 million years ago when the average temperature was about -50°C, to the very hot climate of the Cretaceous Period, when the average temperature exceeded 20°C. More recently, the last few million years have been marked by an alternation of ice ages interspersed with hot interglacial periods. These variations have been attributed to changes in the astronomic parameters that influence the distribution of solar energy received by the Earth's surface. For the last eight thousand years, the Earth has experienced a remarkably stable and hot period, with an average temperature of 15°C that should persist for quite a long time given the stable configuration of its current orbit's astronomical parameters. However, this equilibrium is brought into question by the increase in pressures resulting from human activity. These pressures are visible in numerous studies that agree on a global warming of the temperature of the Earth's surface. In view of this situation, an intergovernmental group, the IPCC1, has been created through the initiative of the WMO2 and UNEP3, in order to regularly carry out an assessment exercise on the climatic conditions of the Earth's surface. The majority of the results discussed below are taken from reports from IPCC and the literature that has contributed to them.

2.2.1. The report on the findings

The combination of long series of temperature measurements on the ground, sea and more recently from space, allows us to plot a global distribution of the changes in surface temperature. Several centers contribute to this collective effort to validate and analyze the data, including the Climate Research Unit (CRU) with the Hadley Center in the Met Office (HadCRUT3), the National Climate Data Center (NCDC) and the Goddard Institute for Space Studies (GISS). These long series are homogenized to correct instrument errors, changes in the observation sites, disturbances linked to the extension of urban heat islands, etc. They are also spatialized, using spatial observations to compensate for lacunae in in situ coverage. The picture that we draw from these time series is very coherent. The conclusion of the IPCC AR44 [SOL 07] is that the Earth's temperature has increased by 0.76 [±0.19]°C since the pre-industrial era (1850-1899) and that the speed of this warming has progressively accelerated, the degree of warming having doubled in the last fifty years in comparison with the last century. The years 2001-2007 have been particularly hot, with an average anomaly of 0.54°C relative to an average calculated over the period 1951-1980. The year 2008 was certainly a little cooler than the years that preceded it, but the increase was still 0.44°C (see http://data.giss.nasa.gov/gistemp/). Since then, global temperatures seem to he stagnating, but it is still too early to determine the exact reasans for this behavior: does this slowing represent a manifestation of the elimate's natural variability, is this a strengthening of the trapping of heat by the ocean, or of the charge in aerosols? The causes of this slowing could be multiple after the marked acceleration in the last decade of the 20th Century and the beginning of the 21st Century. We can see in Figure 2.1 that the warming has not followed a constant trajectory and that its progression has been marked by numerous plateaus.

Figure 2.1. Surface temperafure as a global average for land and sea since 1880 (datafrom http://data.giss.nasa.gov/gistemp/)

The regional distribution is characterized by warm patterns on the continents at high latitudes, the warming being less at low latitudes and over the ocean. Figure 2.2 shows the distribution of anomalies in temperature over two periods of five years, 1998-2002 and 2008-2012 compared to an average distribution over (1951-1980), calculated from data and software available on the site http://data.giss.nasa.gov/gistemp/. Signs of intensified warming at high latitudes ofthe northern hemisphere are very noticeable. We note that even the southern hemisphere shows moderate but significant patterns. It is however necessary to calculate averages over a sufficiently long period of time for a mean figure to emerge since the large natural variability from one year to another can mask signs of warming.

Figure 2.2. Distribution of anomalies in temperature on the Earth's surface for 1988-1992 and 2008-2012 compared to the reference period 1951-1980 (according to http://data.giss.nasa.gov/gistemp/) (see color section)

This warming of the land's surface manifests itself in numerous effects such as the decrease in snow coverage in winter as well as the almost universal melting of glaciers, at low latitudes as well as at temperate latitudes. The rapid melting of the sea ice is equally manifest and its extent has constantly diminished to reach a minimum coverage in September 2007, when it was reduced to 39% less than the average over 1979-2000, a record reached again in 2012.

The world's oceans are warming and the warm patterns, initially observed in the surface layers, are slowly penetrating the interior where they are observed up to a depth of 3,000 m in some regions. However, the distribution of the warm patterns is far from being homogeneous since it depends on the overall circulation of the oceans, which we will discuss later.

2.2.2. Interpretation of the observed changes

The Earth's climate is far from being stable and its history shows a complex evolution. Multiple factors shape it and contribute to its evolution on different time scales. The shape of continents, for example, conditions the circulation of the atmosphere and ocean. The chemical composition of the atmosphere also reflects the slow evolution of the lithosphere and biosphere and contributes to characterizing the climate. On a shorter scale of time, dust emissions due to violent volcanic eruptions that can reach the stratosphere lead to a cooling of the Earth's surface for a few months to a few years. On the scale of the last million years, extensive information taken from polar ice cores and from oceanic sediments demonstrates the importance of the parameters controlling the Earth's orbit around the Sun. If the origin of these oscillations between a dominant glacial climate state and hot interglacial periods is easily identifiable in the difference in distribution of the solar energy received by the Earth's surface, the amplitude of these oscillations can only be explained by implying feedback between greenhouse gases and the climate, variations in solar energy being much too weak to explain the size of the observed variations.

These strong variations in the land climate have been accompanied by changes in polar caps and in sea level. The analysis of air bubbles trapped in polar cores enable the reconstruction of atmospheric concentrations of trace gases; they show important variations with an incidence of elevated concentration rates of carbon dioxide (CO2), methane (CH4) and nitrogen protoxide (N2O) during interglacial periods and low levels in glacial periods. Moreover, these records indicate that current levels of CH4 (1,774 ppb5 in 2006) and CO2(379 ppm6 in 2006) had never been reached during the last million years, which suggests that the current climate has not experienced a parallel during this period.

2.2.3. The Earth's radiative equilibrium and greenhouse gases

The Earth is in a delicate equilibrium between the energy received from the Sun and the energy that it emits into space. It is the small variations in this distribution of energy on the Earth's surface that paleoclimate specialists held responsible for oscillations between glacial and interglacial climates. The Sun, which has a high temperature in the order of 6,000 K emits radiation in the band X and visible spectrum whereas the Earth, whose average temperature is 255 K, mainly emits radiation in infrared wavelengths following Stefan-Boltzmann's law of blackbody radiation [TRE 92]. To say that the Earth is in a radiative equilibrium means that the energy received from the Sun is equal to the energy emitted into space, which leads to an average "radiative" temperature of 255 K (or -18°C). Indeed, the average temperature observed is close to 15°C, or 33°C more than the radiative temperature. This gap is due to the presence of gas in very small quantities (trace gases) whose interaction with different radiative wavelengths significantly alters the energy balance, in quantity according...