Climate Change in Europe and the Role of Internal Climate Variability

The observed changes in climate already have impacts on ecosystems, the economy and on human health in Europe. Risk assessment and regional adaptation strategies need to take in account the role of internal climate variability as its weight over Europe is large with respect to other regions in the world.

By Roberto Barbiero

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One of the most important elements introduced in the latest IPCC WGI AR6 report (Climate Change 2021: The Physical Science Basis) concerns the study of the impacts of climate change at the regional level.

The IPCC report introduces the concept of the Climatic Impact Drivers (CIDs), such as Heat and cold, Precipitation, Drought and aridity, River flood, Fire weather, Wind, Snow and ice and Coastal, considered as physical climate system conditions that affect an element of society or ecosystems. The number of climatic impact-driver changes is expected to increase with increasing global warming and multiple climatic impact-drivers have already changed concurrently over recent decades.

One of the events promoted at COP26 was focused on the most recent regional climate information for the European region based on the latest IPCC WGI AR6 relevant for impact and risk assessment. Future projection of the most relevant CIDs, with their past trend observations and attribution, has been highlighted for each of the 4 European regions considered.

The observed changes in climate are already having wide-ranging impacts on ecosystems, the economy and on human health in Europe. New records continue to be set on European temperatures: 2020 was the warmest year on record, at more than 1.6°C above average. Precipitation patterns are changing. Strong declines in glaciers, permafrost, snow cover extent, and snow seasonal duration at high latitudes and altitudes are observed and will continue. Climate-related extremes such as heat waves, heavy precipitation and droughts, are increasing in frequency and intensity in many regions. Very extreme events have interested Europe in the last months such as severe flooding in Germany and neighbouring countries in july and heat wave, drought and forest fire during summer in Mediterranean regions.

Taking into consideration future scenarios, temperatures will rise in all European areas at a rate exceeding global mean temperature changes. The frequency and intensity of hot extremes, including marine heat waves, are projected to keep increasing regardless of the greenhouse gas emissions scenario. The frequency of cold spells and frost days will decrease under all the greenhouse gas emissions scenarios. An increase in precipitation is expected in Northern Europe while a decrease is projected in summer in the Mediterranean. Extreme precipitation and pluvial flooding are projected to increase at global warming levels exceeding 1.5°C in all regions except the Mediterranean. Regardless of level of global warming, relative sea level will rise in all European areas except the Baltic Sea, at a rate close to or exceeding global mean sea level. 

Focusing attention on Italy, there are two European regions of reference: Western & Central Europe (WCE), which contains the Alpine region, and Mediterranean (MED).

On Western & Central Europe a projected increase in pluvial and river flooding is expected as well as projected increases in hydrological, agricultural and ecological droughts at mid-century warming levels of 2°C or above. On Mediterranean projected increase in hydrological and agricultural and ecological droughts is expected as well as projected increase in aridity and fire weather conditions. Projected combination of climatic impact-driver changes (warming, temperature extremes, increase in droughts and aridity, precipitation decrease, increase in fire weather, mean and extreme sea levels, snow cover decrease, and wind speed decrease) are expected by mid-century and at global warming of at least 2°C and above.

This information on the expected scenarios is fundamental for future risk management to the impacts of climate change, but the researchers’ focus on a factor that makes the reading of future changes more complex for Europe: the role of internal climate variability. Over Europe, accounting for internal variability is essential as its weight is large with respect to other regions.

But what is the internal climate variability? Observed climatic changes since pre-industrial era at any spatial scale are a combination of long-term human caused changes and natural variations on time scales from days to decades. Regardless of future levels of global warming, this combination will continue in the future. Natural variations consist of both natural radiatively forced signals (due to volcanic eruptions or solar variations) and internal fluctuations of the climate system, which occur spontaneously. Since preindustrial period, natural climate variability have temporarily obscured and intensified human caused climate change on interannual to decadal time scales.

The internal variability plays an important role for Europe causing a certain delay in defining what is called “time of emergence”, that is the date from which the new mean climate corresponds to unfamiliar conditions in preindustrial period. These time of emergencies are “delayed” over Europe with a latitudinal gradient due to greater weight of internal variability going north despite stronger warming. Emergencies in the climate system emerged in 1981-1988 for Mediterranean but later in 2005-2012 for Northern Europe. The weight of internal variability differs not only between regions, but also between climate variables, for precipitation more than temperature, and between seasons, for winter more than summer.

Natural drivers and internal variability will either amplify or attenuate projected human caused changes in mean climate and climatic impact drivers (CIDs), including extremes, especially at regional scales and in the near-term (2020-2040), but with little effect on centennial global warming. 

Modulations driven by internal variability are important to consider in planning for the full ranges of possible changes for risk assessment and regional adaptation strategies in Europe.