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Climate in a narrow sense is usually defined as the average weather, or more rigorously as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. Climate encompasses patterns of temperature, precipitation, humidity, wind and seasons. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization (https://public.wmo.int/en). Our climate is now changing at a rate faster than any seen in the last 2000 years (www.ecology.wa.gov). This is due to rising levels of carbon dioxide and other heat‐trapping (greenhouse) gases in the atmosphere. The primary greenhouse gases in Earth’s atmosphere are water vapour (H₂O), carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄) and ozone (O₃). These act like a blanket around Earth, causing it to warm – a phenomenon referred to as ‘global warming’. According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by about 0.8 °C since 1880. Two‐thirds of the warming has occurred since 1975, at a rate of roughly 0.15–0.20 °C per decade (Figure 3.15). Wide‐ranging impacts of rising temperatures include rising sea levels, melting snow and ice, more extreme heat events, fires and drought and more extreme storms, rainfall and floods. This rise in global average temperature is attributed to an increase in greenhouse gas emissions. The link between global temperatures and greenhouse gas concentrations, especially of CO₂, has held throughout Earth’s history (Lacis et al. 2010). CO₂ concentrations in the atmosphere are now well over 400 ppm, their highest levels in over 800 000 years (Figure 3.16). Globally, we emit over 36 billion tonnes of CO₂ per year, and this continues to increase. There are large differences – more than 100‐fold – in per capita CO₂ emissions between countries (Richie & Roser 2020). For example, the United States has contributed most to global CO₂ emissions to date, accounting for 25% of cumulative emissions. This is followed by the EU‐28 (22%), China (13%), Russia (6%) and Japan (4%). There are large inequalities in CO₂ emissions: the world’s poorest countries have contributed less than 1% of emissions, but will be the most vulnerable to climate change impacts. Rising levels of atmospheric CO₂ is increasing ocean acidity, which has been predicted to double over the next 100 years in an uncontrolled emission scenario (Fauville et al. 2013). Approximately one‐third of the anthropogenic carbon added to the atmosphere is absorbed by the oceans. Uptake of atmospheric CO₂ results in a decrease in ocean water pH, an effect referred to as ‘ocean acidification’ (OA; see later).

The Intergovernmental Panel on Climate Change (IPCC) is the UN body responsible for assessing the science related to climate change. It was established by the United Nations Environment Programme and the World Meteorological Organization (WMO) in 1988 to provide policymakers with regular scientific assessments concerning climate change, its implications and its potential future risks, as well as to put forward adaptation and mitigation strategies. The IPCC’s reports are comprehensive and balanced assessments of the state of knowledge on topics related to climate change. There are different types of reports, but all go through a rigorous process of scoping, drafting and review to ensure the highest quality. To date, the IPCC has 195 member states. The Paris Agreement is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC) dealing with greenhouse gas emissions mitigation, adaptation and finance. As of November 2019, 190 of 195 IPCC member countries have signed the Agreement. Its long‐term temperature goal is to keep the increase in global average temperature to well below 2 °C above pre‐industrial levels and to pursue efforts to limit the increase to 1.5 °C, recognising that this would substantially reduce the risks and impacts of climate change. The Paris Agreement was intended to become fully effective in 2020. Predictions of global environmental conditions for the end of the century coupled with ever‐increasing experimental evidence suggest wide‐ranging impacts of future OA and warming scenarios on marine life (Poloczanska et al. 2016). Humans, through activities such as burning fossil fuels, deforestation, industrial processes and some agricultural practices, are largely responsible.

Figure 3.15 Global average temperature for the period 1880–2018. While global average temperatures vary from year to year, the overall trend from 1880 to the present is one of increased temperature.

Source: Data from National Centers for Environmental Information.

https://www.ncdc.noaa.gov/cag/global/time‐series/globe/land_ocean/1/1/1880‐2020.

Figure 3.16 Global atmospheric carbon dioxide (CO₂) concentrations in parts per million (ppm) for the past 800 000 years. The peaks and valleys track ice ages (low CO₂) and warmer interglacials (higher CO₂). During these cycles, CO₂ was never higher than 300 ppm. In 2018, it reached 407.4 ppm. On the geologic time scale, the increase (dashed line) looks virtually instantaneous.

Source: Data from Lindsey (2020).

So, what are the consequences of climate change for marine ecosystems? Mora et al. (2013), using global climate models, have shown that in the next 100 years, the entire world’s ocean surface will be simultaneously impacted by varying intensities of ocean warming, acidification, oxygen depletion or shortfalls in productivity. In contrast, only a very small fraction of the world’s ocean surface, mostly in polar regions, will experience increased oxygenation and productivity, and almost nowhere will there be cooling or pH increase. From a compiled list of 32 marine habitats and biodiversity hot spots, Mora et al. (2013) found that all would experience simultaneous exposure to changes in multiple biogeochemical parameters, which will demand multiple physiological adjustments from marine biota. However, regional‐scale differences in response to climate change can often be more relevant than global averages. For example, a study of SST change in 63 global large marine ecosystems (LMEs) over a 50‐year period (1957–2006) revealed strong regional variation, with the Subarctic Gyre, European Seas and East Asian Seas warming at two to four times the global mean rate (Belkin 2009). The Subarctic Gyre warming is likely caused by natural variability in relation to the North Atlantic Oscillation, a climatic phenomenon which varies over time but has no particular periodicity. The most rapid warming was observed in the land‐locked or semi‐enclosed European and East Asian Seas (Baltic and North Seas, Black Sea, Japan Sea/East Sea and East China Sea), and also over the Newfoundland–Labrador Shelf. The proximity of the European and East Asian Seas to major industrial/population agglomerations suggests a possible direct anthropogenic effect. In a comparable study, Alexander et al. (2018) examined changes in SSTs in 18 LMEs adjacent to North America, Europe and the Arctic Ocean. The annual SST trends over 1976–2099 in all 18 were positive, ranging from 0.05 to 0.5 °C per decade. SST changes by the end of the 21st century will primarily be due to a positive shift in the mean, with only modest changes in the variability in most LMEs, resulting in a substantial increase in warm extremes and decrease in cold extremes. The shift in the mean is so large that in many regions SSTs during 2070–2099 will always be warmer than the warmest year during 1976–2005. The SST trends are generally stronger in summer than in winter, which amplifies the seasonal cycle of SST over the 21st century. In the Arctic, the mean SST and its variability increase substantially during summer, when it is ice‐free, but not during winter, when a thin layer of ice reforms and SSTs remain near the freezing point. While basin‐wide changes in the ocean are expected (Alexander et al. 2018), it is critical to examine temperature changes along continental margins, which supply more than 75% of the world’s marine fish catch. Lima & Wethey (2012) explored global and monthly warming patterns along 19 276 coastal locations between 1982 and 2010. They demonstrated that 46% of the coastlines had experienced a significant decrease in the frequency of extremely cold events, while extremely hot days were becoming more common in 38%. They further showed that the onset of the warm season was advancing significantly earlier in the year in 36% of the temperate coastal regions. More importantly, it is now possible to analyse local patterns within the global context, which is useful for a broad array of scientific fields, policymakers and the general public. Li et al. (2019) similarly conducted a global analysis of SST at 26 locations in Chinese coastal waters.