The Earth’s climate has changed throughout history, about 7 cycles in the last 650,000 years with the abrupt end of the last ice age about 7,000 years ago, marking the beginning of the modern climate era and of human civilisation. Most of these climate changes have been attributed to variations in Earth’s orbit modifying the amount of solar energy received at the Earth’s surface.
Currently, anthropogenic climate change is receiving much attention, but is leading to controversial political and societal debates, as it would require major changes in our societies (which are also more vulnerable than in the past) in a short time period to mitigate it. The main evidence of the anthropogenic global change is the warming temperature: the planet’s average surface temperature has risen about 1.1 ◦C since the late 19th century (IPCC, 2014). Most of the warming occurred in the past 35 years. The main cause is well acknowledged by all the scientific expert of the climate system: the increase of green-house gas concentrations in the troposphere, in particular CO2 – Carbon dioxide and CH4 – Methane that are responsible for approximately 80% of the observed radiative forcing (IPCC, 2014).
In addition to CO2 and CH4, also other greenhouse gases contribute to the warming climate such as H2O (as a feedback of temperature rise), N2O, CFCs and O3. The basic processes governing the greenhouse effect, i.e. transmission of solar radiation but absorption of thermal infrared, was already identified during the 18th century. It is characterized by warming the lower atmosphere and cooling the upper atmospheric layers. Initially, the green-house effect is not only natural but also necessary for developing life on Earth. Indeed, without it, the average temperature would be too cold. Later, the scenario of an excessive global warming caused by the injection of gases such as CO2 was discussed by Arrhenius (1896).
Other compelling evidences suggest additional rapid changes in many more variables that characterise the state of the whole climate system. These changes are then beyond air temperature rise:
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Warming oceans – Oceans also absorb part of the enhanced heat balance in the atmosphere. As a consequence, increased heat has also been absorbed within the top 700 m of the ocean surface (Levitus et al., 2009).
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Ocean acidification – Oceans have absorbed about 25 % of emitted anthro- pogenic CO2. Due to this increase, acidity of surface ocean waters has increased by about 30% (Levitus et al., 2009).
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Shrinking ice sheet and declining sea ice – Greenland lost 150 to 250 km3 of ice per year between 2002 and 2006, while Antarctica lost about 150 km3 of ice between 2002 and 2005. Both the extent and thickness of Arctic sea ice has rapidly declined over the last decades.
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Glacial retreat – Everywhere in the world, including in the Alps, Himalayas, Andes, Rockies, Alaska and Africa, glaciers are slowly disappearing.
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Decreased snow cover – Satellite data have revealed that snow is melting earlier and the amount of spring snow cover has decreased over the past five decades in the North Hemisphere.
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Sea level rise – The pace of sea level rise has nearly tripled since the last two decades. Current estimations are about 1.5 mm per year globally between 1958 and 2014 (Frederikse et al., 2017a). However, such an increase is highly spa- tially heterogenous: e.g. about 1.6 mm per year in the North-Sea while it rises faster on the US Atlantic Coast about 3.4 mm per year since 1965 (Frederikse et al., 2016, 2017b).
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Extreme events – In some parts of the world, such as in the USA, there is an increase of intense rainfall events and storms due to an enhanced record high temperature episodes and a decreased number of record low temperature events (Kunkel et al., 2013). Furthermore, there is generally an increase of wildfires throughout the world, particularly in Canada British Colombia, east Russia, Europe and lately in December 2017 in California region. This is a result of dry forests and warmer temperatures.
In addition to the greenhouse gases, aerosols are also an important component of climate. As they scatter and absorb the sunlight, they redistribute shortwave radiation in the atmosphere. Aerosols are an important player in the climate system by leading, on average, to surface cooling and additional atmospheric dynamical re- sponses (Boucher et al., IPCC report, Chapter 5: Clouds and aerosols, 2015). By acting as the condensation nuclei on which clouds form, they also modify cloud formation, lifetime and precipitation (Figueras i Ventura and Russchenberg, 2009; Sarna and Russchenberg, 2017).
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