Température moyenne à la surface de la terre

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Global climate change is a surtout concern. It is based on the clinical community’s statement that human activities are the causer of a present et future globale warming. Marqué can nous really define thé average temperature of the planet and, if so, measure it? Can we really achieve année accuracy of a couple of tenths du a degree when we give auto warming trends of the last century or when the international community set a target ns limiting warming to seulement un 2°C or even 1.5°C? Here we address thé question ns methodologies parce que le determining thé average temperature at the Earth’s surface, the accuracy of current reconstructions, et the questions du the observation and causes de its recent evolution.

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1. Does thé average global temperature make sense?

1.1. Link between energy balance et global temperature

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Figure 1. Average energy balance pour the period 2000-2005 (in W/m2). The arrows represent the energy flows in proportion à their intensities. Auto balance imbalance term de 0.6 W/m2 is constant with observed alters in the ocean’s energy content. >The climate système (atmosphere, ocean, ice sheets, vegetation, etc.) obtain most ns its power from thé Sun (Figure 1) <1>. This power received in auto form ns radiation corresponds roughly to the radiation of a le noir body with a temperature of about 5,800 K (Read black Body heat Radiation), greatly in thé visible range. Oui it passes through thé atmosphere, some du this radiation is reflected or scattered de clouds, airborne particles (aerosols) and atmospheric gases. Un autre part du this radiation is absorbed passant par these gases et aerosols. Seul part du the solar radiation reaches the surface, where it is also partly reflected. Therefore, seul about half ns the solar radiation incident at the top de the atmosphere (340W/m2) is took in at thé Earth’s région (160W/m2).

This power input pour the oceans and continents is, at equilibrium, compensated by an equivalent power loss. This lose takes carré either in the form du radiative energy, or in the form de heat move linked venir conduction (known as sensible heat) or linked to étape changes in thé water (known as latent heat). These energy losses are toutes les personnes a function de the Earth’s temperature, in particular auto radiation emitted passant par the surface, which is fermé to that of a 288 K black body et is because of this in auto infrared range.

It is to trop sale towards this energy balance at the surface, but also at the top of the atmosphere, that thé Earth’s temperature is changing. Marqué even if this physical mechanism is well known, the imaginations of a global average de the Earth’s temperature is not easy venir grasp.

1.2. Median temperature? A statistics indicator!

The temperature ns a solid, liquid jaune gaseous milieu is a physical amount that reflects the agitation du the particles the make cette up in a given place. The sum du two temperatures therefore has no physical meaning. As a result, an average temperature end a wide variety over which ce varies from une place to autre has no direct physical interpretation. This is therefore the boîte for thé average temperature calculated over auto entire surface of the Earth, covering both auto warm dry regions et the cold polar regions. However, this average is a statistical indicator that proves really useful pour assessing climate change on a global scale, both in the past et in projection parce que le the comes centuries. The globale average temperature reflects transforms in climate that have the right to be explained de identifiable underlying physical mechanisms.

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1.3. Instance 1: dommage of greenhouse gases on the Earth’s mean temperature

A first example is thé average temperature difference that deserve to be estimated passant par calculating auto Earth’s power balance due to auto presence du naturally arising greenhouse gases in auto atmosphere. This gases oui the property de absorbing auto infrared radiation emitted at auto Earth’s surface et then re-emitting some of cette back to thé surface, thereby additional warming cette (Figure 1). These incorporate water vapour, carbon dioxide (CO2) and methane (CH4) à name the main ones. Their contribuant to the average globale temperature is of the order of 33°C warming effect, pass this temperature from around -18°C à +15°C.

It need to be provided that this calculation only makes sense “all jambe being equal”, because si the Earth’s temperature were à cool de about 30 degrees, a change in the ice cover on its surface would an outcome in extr cooling tandis que to enhanced reflection du solar radiation (ice-albedo feedback effect).

1.4. Instance 2: Effect of the Earth’s orbital parameters nous its typical temperature

Another example ns a physical procedure resulting in a large contourne in the global mean temperature is primarily due to monnaie in auto eccentricity ns the Earth’s orbit. Auto Earth’s orbit around the sun is an ellipse who eccentricity, measure up the détour from its shape à that ns a circle, varies between 0 (circular orbit) and 0.06 over auto last million years through a henchmen cycle of about 100,000 years. Ont eccentricity increases, thé average supprimer from the earth to thé sun increases et thus auto solar radiative power received passant par our world decreases. Thé result is a 100,000-year climate cycle du alternating cold (glacial) et warm (interglacial) durations over thé last million years or so.

Understanding this 100,000-year période is calmer being researched since the habitent effects du variations in auto amount of energy received by the terre are small. Some apprendre <2> seul that it is additionally necessary to take right into account auto effects ns variations in est différent astronomical parameters (obliquity, precession; see animations The drivers de natural climate evolution) et the physical impacts amplifying temperature differences between cold et hot periods :

The ice-albedo feedback effect currently mentioned: thé decrease in la glace cover during warm periods reduces auto reflectivity of solar radiation (the albedo) at thé Earth’s surface, i m sorry contributes venir increased warming par absorbing more radiative energy.The greenhouse effect because warm durations are likewise periods throughout which greenhouse gas concentrations increase as a result de physical processes and ecosystem changes.

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The resulting globale average temperature distinction between thé last cold duration at its extreme (the last déplacement maximum) about 21,000 years ago and the warmth interglacial duration we have known parce que le about 10,000 year is therefore likely between 3°C and 8°C <3>.

Animation “The drivers de climate change” (Credit: Museum aux Toulouse, Mercator Océan)