The Evidence for Anthropogenic Climate Change
1 Introduction
Climate change has been a hotly debated topic in recent years with the effects of it clearly felt all around the world. Global temperatures have been increasing by 0.08°C per decade since 1880 (NOAA, 2020). Most of this warming has occurred in recent decades, with 10 of the warmest years on record occurring since 2000. This has serious implications for the global climate which would lead to the warming and acidification of the oceans, rising sea levels (IPCC, 2014a), the loss of polar ice sheets (Velicogna et al., 2020) and the loss of sea ice that leads to Arctic Amplification (Serreze et al., 2009).
The Intergovernmental Panel on Climate Change (IPCC) has suggested in recent times that it is extremely likely (>95% confidence) that this warming trend is the result of human activity (Solomon et al., 2007). Furthermore, the general consensus amongst research scientists on anthropogenic induced warming is 100% (Powell, 2019). However, the climate also experiences natural variations which causes changes in the climate that originate from natural sources, albeit at varying time scales. This is known as natural variability and is influenced by internal and external processes within the climate systems. Some of these processes include natural cycles such as the Milankovitch Cycle, variations in solar irradiance and decadal or multidecadal oscillation patterns and can result in the warming or cooling of the climate (Beer et al., 2000). Therefore, in order to accurately quantify the impact of anthropogenic emissions on global warming, it is important to account of natural variability.
Given the overwhelming scientific consensus on anthropogenic global warming, it is important to review the evidence that supports this consensus. This article seeks to identify and elaborate on the evidence for anthropogenic climate change by reviewing literature on radiative forcing, carbon isotopic signatures, paleoclimatology and natural variability.
2 Radiative Forcing of Greenhouse Gases
Earth has a natural greenhouse effect, which allows for the absorption of solar radiation in our atmosphere and creates a warming effect (Anderson et al., 2016). Greenhouse gases, such as carbon dioxide and water vapour, are the main drivers behind this greenhouse effect. Conversely, aerosols within the atmosphere create a cooling effect by scattering incoming solar radiation (IPCC, 2014b). These warming and cooling effects are the result of positive and negative radiative forcing exerted by forcing agents (e.g., greenhouse gases, aerosols), which can be naturally or anthropogenically sourced. The IPCC (2014b) has indicated recently that anthropogenic greenhouse gases have resulted in a positive radiative forcing on the Earth’s climate. This is summarized in Figure 1, which reflects the cumulative radiative forcing of anthropogenic and natural forcing agents within the period of 1750 and 2011.
The evidence provided by the IPCC suggest that the recent acceleration of the warming trend is the result of a positive radiative forcing from anthropogenic sources, with greenhouse gases such as carbon dioxide (CO2) having the largest contribution. It is interesting to note that radiative forcing due to natural solar irradiance results in a much lower positive radiative forcing compared to greenhouse gases, suggesting that variations in solar irradiance are not sufficient to explain the significant warming trend experienced. Because an increase in anthropogenic greenhouse gases results in a strong positive radiative forcing, it can be concluded that the recent warming of the Earth’s climate is a direct result of an increase in anthropogenic greenhouse gases.
3 Isotopic Signatures
Human impacts on the environment can be tracked through selected isotopic data stored within natural archives. Isotopes are variants of an element that differ in the number of neutrons (Hoefs, 2009), whereby environmental factors can result in changes in the isotopic ratio of an element. These are especially useful as they allow for the reconstruction of paleo environments such as the climate and composition of the Earth’s atmosphere.
As mentioned in the previous section, an increase in carbon dioxide concentrations in the atmosphere is one of the main drivers behind the recent warming trend (Anderson et al., 2016). This trend is reflected in the δ13C and δ13CO2 isotopic records as illustrated in Figure 2, which was obtained from Antarctic ice cores, foraminifera and observational records from the observatory at Mauna Loa (Dean et al., 2014).
Since 1750, commonly termed as the start of the industrial revolution, δ13CO2 has been declining (having a more negative ‰), with an acceleration of the decline occurring in the 1950s. This coincides with a more recent increase in atmospheric CO2 concentrations from 280 ppm in 1750 (Meure et al., 2006) to 316ppm in 1959 and to 396 ppm in 2013 (Deanet al., 2014). The accelerating trend of atmospheric CO2 concentrations has a strong correlation with δ13CO2 values as observed in the declining trend between 1958 and 2013 in Figure 2. The decline in the δ13CO2 values is known as the Suess effect and is driven by CO2 emissions derived from burning of fossil fuels (Keeling et al., 2017). The reason for this is as such. Fossil fuels originate from organic sources (such as plants) and are depleted in 13C isotopes due to the fractionation of isotopes during photosynthesis. This results in fossil fuels having a low δ13C value. Burning these fossil fuels will result in CO2 emissions which are isotopically lighter and depleted in 13C isotopes as compared to atmospheric CO2 (Keeling, 1979), thereby lowering δ13CO2 and δ13C values (Keeling et al., 2017).
Isotopic records of atmospheric concentrations of CO2 also include many other sources such as gas bubbles of CO2 trapped in ice cores (Bauska et al., 2015), coral samples (Swart et al., 2010) and long-lived bivalve molluscs (Butler et al., 2009). However, some of these records have varying time scales with organic samples (e.g., coral and molluscs) often having a much shorter time scale (200 year record). These natural archives all record the global anthropogenic impact within its isotopic data and provides conclusive evidence that CO2 emissions from anthropogenic sources (specifically the burning of fossil fuels) results in an increase in atmospheric CO2 concentration and ultimately, global warming.
4 Earth’s Paleoclimate
Over the past few centuries, the Earth has experienced a significant increase in the atmospheric CO2 concentrations, which is largely attributable to the burning of fossil fuels. To put the significance of this finding into context, scientists have looked at Earth’s paleoclimate over the past 800,000 years to identify trends in Earth’s atmospheric composition. Over the past 800,000 years, 11 interglacial periods occurred (Berger et al., 2016). This is well documented in isotopic records obtained from gas bubbles trapped within the Antarctic ice cores and is illustrated in Figure 3.
Lüthi et al. (2008) suggest that atmospheric carbon dioxide concentration is strongly correlated with Antarctic temperature, with carbon dioxide concentrations over the last 800,000 years fluctuating between a range of 172 ppm — 300 ppm (Figure 3). Atmospheric CO2 concentrations are low during a glacial period (periods with significant negative temperature anomalies) because oceans are colder, which absorbs a higher concentration of atmospheric CO2 (Sigman and Boyle, 2000). The expansion of sea ice and the reduction of sea levels during glacial periods also play a role in reducing CO2 concentrations, as these factors reduce the release and exchange of CO2 from the ocean to the atmosphere (Sigman and Boyle, 2000). Looking at the paleoclimate of the past 800,000 years, it is clear that an increase in atmospheric CO2 concentration would lead to an increase in temperature. As such, it is not a surprise to see that recent warming trend is complemented by a significant increase in atmospheric CO2 concentration. It is interesting to note that the glacial periods recorded span illustrate a cyclical pattern with the glacial periods having a time scale on the order of 100,000 years (Berger et al., 2016). This suggests that the glacial periods are likely driven by periodic changes in the Earth’s eccentricity, a phenomena widely known as the Milankovitch Cycles. The changes in the Earth’s orbital cycles result in changes in the distribution of incoming solar radiation.
Earth’s paleoclimate over the past 800,000 years suggest that increasing atmospheric CO2 concentrations are natural and largely due to variations in the Earth’s orbital cycles. However, this explanation cannot sufficiently explain the recent increase in atmospheric CO2 concentrations to the highest levels observed in over 800,000 years (Keeling et al., 2005). There are two major findings that disprove the notion that the recent warming trend is due to orbital cycle variations. The first is that atmospheric CO2 concentrations over the past 800,000 years have never exceeded 300 ppm. The second is the rate at which atmospheric CO2 concentrations have increased since the industrial revolution, which increased significantly over two and a half centuries as opposed to over a few millennia (as is the case during interglacial periods). Through the study of Earth’s paleoclimate, it can be concluded that the recent rise in atmospheric CO2 concentrations since the industrial revolution is less likely to be the result of natural external forcing.
5 Natural Variability
The Earth’s climate fluctuates naturally with variations in its climate parameters. This is known as natural variability, which is influenced by natural internal or external processes within the climate system. External processes that affect the climate system include variations in solar irradiance and variations in Earth’s orbit (e.g., Milankovitch Cycles) whereas internal processes are processes which arise from changes in ocean circulation and interactions between the atmosphere and ocean (Swanson et al., 2009). The timescales at which the external and internal processes occur varies, with some external processes occurring on much longer timescales. The variation in Earth’s eccentricity, which was mentioned in the previous section, and solar variability are examples of natural external variability.
In order to conclusively ascertain the role of anthropogenic CO2 in global warming, the natural variability of the system should be accounted for. This is because natural variability plays a role in global warming and can result in changes in atmospheric CO2 concentrations. Scafetta and West (2006) find that solar variability, which periodically occurs on an 11-year cycle, accounts for about 50% of global warming in the 20th century and about 30% of global warming from 1980–2000. This suggests that natural external variability plays a significant role, albeit slightly diminished in recent decades, in global warming.
In light of the recent global warming slowdown from 2000–2014 (Easterling and Wehner, 2009), greater attention has been brought to address the role of natural internal variability. This was significant as the global warming slowdown occurred during a time when atmospheric CO2 concentrations were continuously increasing. As such, it was concluded that natural internal variability likely played a major role in this slowdown (Wu et al., 2019; Watanabe et al., 2014). Natural internal variability of the climate system can be generated by, but are not limited to, multiple oscillation patterns such as the El Nino Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). These oscillation patterns are recurring patterns and variations of ocean-atmosphere climate variability that often bring about anomalies in local temperature and are centered over various geographic locations around the world. These oscillation patterns occur over varying timescales. For example, the PDO is a pattern that occurs over inter-annual or inter-decadal timescales while the AMO occurs over multidecadal timescales and ENSO occurs over interannual timescale. Wu et al. (2019) show that between 1880 and 2017, natural variability over the Pacific and Atlantic, due to the PDO and AMO respectively, accounted for 30% of warming experienced. Anthropogenic greenhouse gases accounted for the remaining 70%. While the bulk of warming experienced was attributed to anthropogenic greenhouse gases, natural variability cannot be ignored due to the significance it plays.
Unfortunately, the role of natural variability in climate change is not sufficiently quantified and a research gap exists in understanding and quantifying the climate’s natural variability. The role of natural variability is especially important as it allows us to understand the true impact of anthropogenic emissions and allows for an accurate modelling of Earth’s paleoclimate. Furthermore, natural variability is important for climate model projections in order to accurately predict the future impact of anthropogenic emissions based on various emission scenarios. A possible avenue for further research could be to evaluate and quantify the impact of natural variability and use climate models under the Coupled Model Intercomparison Project Phase 6 (CMIP6) to predict the impact of future anthropogenic emissions on global warming and climate change (Eyring et al., 2016).
6 Conclusion
There is strong evidence to suggest that the recent global warming trend in recent decades is the result of anthropogenic greenhouse gases. Since the start of the industrial revolution, anthropogenic greenhouse gases (predominantly CO2 and CH4) have strongly contributed to a positive radiative forcing of the climate. This has coincided with a rapid increase in global temperature and a rapidly changing climate. It is known that these greenhouse gases are anthropogenically induced due to the isotopic signature of carbon in the atmosphere, which has become isotopically lighter and depleted in 13C isotopes in recent decades. This is due to the burning of fossil fuels that release isotopically lighter 13C isotopes into the atmosphere. A review of Earth’s paleoclimate also suggest that the increase in atmospheric CO2 since 1750 is significant because Earth’s atmospheric CO2 concentration has never once passed the threshold of 300 ppm over the past 800,000 years. Yet the current atmospheric CO2 concentration has risen significantly to over 396 ppm as of 2017 (Dean et al., 2014).
Although global warming is primarily driven by anthropogenic greenhouse gas emissions, natural variability also plays a role in the warming trend observed. It is suggested that between 1880 and 2017, natural variability accounted for 30% of the warming experienced while anthropogenic greenhouse gas emissions accounted for 70% (Wu et al., 2019). While natural variability can explain some of the changes observed in the today’s climate, its impacts are less significant than that of anthropogenic greenhouse gas emissions and therefore cannot entirely be the cause of climate change. Because natural variability of the climate can enhance or reduce global warming, it should therefore be accounted for in climate projections to accurately predict the impact of anthropogenic greenhouse gas emission.
Amongst the scientific community, there is a 100% consensus on anthropogenic-induced climate change. This article provides evidence to substantiate that consensus and places it in the context of natural variability. Anthropogenic greenhouse gas emissions are the drivers behind the changes in our climate, changes that natural variability can no longer explain. Therefore, greater attention should be given to the reduction of anthropogenic greenhouse gas emissions and its impacts even as the world continues to develop and pursue economic growth.
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