The role of biology in global climate change

Abstract

Biogeochemistry aims to understand the flux of elements between life, the atmosphere and the Earth's surface. Its insights could inform international policies to mitigate the effects of greenhouse gases and global climate change.

Research in biogeochemistry has expanded rapidly over recent years as the field could provide answers to major scientific and societal challenges such as climate change, contamination of groundwater, and food security. Yet it dates back almost a century: Its origins are usually attributed to Ukrainian scientist Vladimir Vernadsky whose 1926 book The Biosphere proposed that the Earth's surface could not be understood in terms of pure geochemistry unlike Mars or Venus or other planets, but required full integration of life. The key point, popularized around half a century later by James Lovelock with his controversial Gaia Hypothesis, was that living organisms are integral to the chemistry of the planet surface and inseparable from it. While the strong version of Gaia, that the surface of the planet can itself be regarded as a super self‐regulating organism subject to evolution, is rejected by most scientists, the fundamental idea that life has imbued every aspect of surface chemistry and left its fingerprints within the Earth's mantle has become mainstream.

… the fundamental idea that life has imbued every aspect of surface chemistry and left its fingerprints within the Earth's mantle, has become mainstream.

Biogeochemistry is now firmly rooted on fundamental principles that govern the global cycles of the four most important elements for life: carbon, oxygen, nitrogen, and phosphorus, along with other minerals that participate in surface chemistry. The “natural” movement of those four elements—as well as major elements with toxic properties such as lead, mercury, arsenic, and cadmium—has been augmented by human activities such as mining and industrial production. This added human dimension lies at the heart of applied biogeochemistry and has helped provoke an upsurge in research, according to William Schlesinger, recently president of the Cary Institute of Ecosystem Studies. “I think there are two reasons. First, we are beginning to realize that changes in the Earth's chemistry underlie many of the major environmental problems that we face, including the ozone hole, global climate change, hypoxic zones in coastal waters and arsenic poisoning of groundwater”, he said. “Second, we are increasingly realizing that biological processes affect and even dominate the chemistry at the surface of the Earth. What we once called geochemistry now needs to be biogeochemistry”.

Global cycles of elements

In fact, the original transformation from geochemistry to biogeochemistry started about 2.3 billion years ago with the Great Oxygenation Effect (GOE) when oceanic cyanobacteria evolved into multicellular forms that liberated oxygen more efficiently. Oxygen sinks such as dissolved iron or organic matter eventually became saturated and di‐oxygen produced by cyanobacteria started to accumulate in the atmosphere which, over a 900‐million‐year period, generated the reduction mechanisms that control Earth's geochemistry and biochemistry today.

The global cycles of carbon and oxygen dominate all other elements of life in quantitative terms, but they are intricately linked with nitrogen and phosphorus that play key roles as limiting factors. Recent research into these cycles has further highlighted how interdependent biology and geology are, according to Ben Houlton, Principal Investigator for Earth Systems Ecology and Biogeochemistry at University of California, Davis, whose team showed that rocks are a significant source of nitrogen to ecosystems 1. “This rock nitrogen input was previously unrecognized by biogeochemical models. Given that nitrogen is one of the most limiting resources to the productivity of ecosystems, this work has implications for how much carbon dioxide can be stored on the land naturally”, he explained. “It opens biogeochemistry to a new realm of questioning, linking geo‐biology, plate tectonics, weathering and ecology over both short and long time scales”.

…we are increasingly realizing that biological processes affect and even dominate the chemistry at the surface of the Earth.

The work on rock nitrogen specifically has some interesting implications for the carbon cycle since rocks act as a natural, slow‐release fertilizer that boost CO₂ uptake by ecosystems, Houlton argued. “We have estimated that sites with relatively high rock nitrogen concentrations store 50% more carbon in living biomass and soil”, he said. “We are beginning to see a strong connection between biological diversity and geological diversity in maintaining the functioning of the biosphere. We estimate that rock nitrogen sources are widely distributed—from the poles to the tropics—and chemically active and mobile in terrestrial ecosystems”.

… replanting is merely putting back carbon that was taken out by past deforestation and cannot make up for carbon release from fossil fuels.

Such work adds finer details to the basic principles of biogeochemistry, according to Schlesinger: “The details of what controls the processing and flow of materials on land and in the sea, will increasingly require ‘deconstructing’ these systems, and using new tools such as molecular biology to elucidate them”. It has already yielded enough information about the intricate relationship between the Earth's surface, its atmosphere and life to guide global policy on a variety of key issues, in particular climate change and mitigation of greenhouse gas emissions.

Greenhouse gases

Methane is the more potent greenhouse gas, absorbing more emitted terrestrial radiation per molecule, but there is 200 times as much CO₂ in the atmosphere at 380 parts per million (ppm) against 1.75 ppm methane. This makes CO₂ the biggest contributor to anthropogenic warming by calculating the total amounts of each gas added to the atmosphere since pre‐industrial times and multiplying it by its global warming potential per unit mass. On that basis, the latest, widely accepted estimate by the Carbon Dioxide Information Analysis Center of the US Department of Energy (DOE) ranks CO₂ number one at 57.8%, followed by methane at 14.9%, ozone at 11.9%, nitrous oxide at 5.9%, and the rest 9.5% (http://cdiac.esd.ornl.gov/pns/current_ghg.html).

As the biggest contributor, CO₂ has received the greatest attention with the creation of elaborate accounting and offset schemes that have come in for criticism as a result of ongoing research. The main point of contention regards the relationship between emissions from fossil fuel combustion and the land‐based carbon cycle. Human activities, notably deforestation, have also increased atmospheric CO₂ concentration, but in those cases, it is a two‐way process with scope for remediation. This has been reflected in trading schemes, whereby planting trees can be used to offset fossil fuel emissions for individual countries in meeting carbon targets.

However, this approach is fundamentally flawed scientifically, according to a 2013 article, which argues that the capacity of terrestrial ecosystems to store carbon is finite and that the current sequestration potential primarily reflects depletion owing to past deforestation 2. In other words, replanting is merely putting back carbon that was taken out by past deforestation and cannot make up for carbon release from fossil fuels. Furthermore, the two sources of CO₂ are subject to different forces and so cannot be lumped together: Climate change itself potentially affects the land‐based carbon budget, while direct human activities impinge on fossil fuel consumption. “The prime implication is that fossil fuel emissions cannot be offset by reforestation, contrary to popular belief and as allowed by international and national policy”, commented the paper's lead author Brendan Mackey, Director of the Griffith Climate Change Response Program at Griffith University, Australia.

As Mackey explained, climate change might increase potential carbon stocks in some regions by, for example, raising rainfall, or lowering evaporation in places where plant growth is limited by water availability. Increases in temperature might also boost the growing season in temperate regions and increase carbon capacity that way. On the other hand, increasing aridity in some tropical regions including the drier rain forests is likely to reduce plant growth. The underlying point is that such competing processes result in changes in the land carbon stock, which in turn are driven ultimately by release of CO₂ from fossil fuels, so that it makes little sense to trade the two sources off against each other.

With little sign that these ideas are being taken up by decision makers, Mackey and his colleagues have followed up with more specific recommendations. “I see our two main policy points being firstly that fossil fuel emission cannot be offset, and secondly, avoiding emissions through forest protection is a critical part of the solution”, he said. “The issue is that people have to understand that fossil fuel and ecosystem carbon are two distinct carbon stocks. The policy makers, however, seem content with allowing fossil fuel emissions to be offset in national greenhouse gas inventories, so‐called net accounting, by using carbon credits generated in the land sector, including through changing forest management. The Paris Agreement now enables the international transfer of such credits (i.e., carbon trading). The lack of what I consider to be science‐based carbon mitigation policy can perhaps be explained by a desire on the part of governments to fiddle at the edges and to be seen to be making incremental progress while maintaining business‐as‐usual approaches”.

Sources of methane emissions

Meanwhile there has also been progress in understanding the role of human activities in the release of methane, the second most important greenhouse gas. The main challenge here lies in determining the relative contributions of the various sources, both human and natural. The gas can be emitted as a result of warming itself through the impact on methane hydrates on the ocean bottom, which are cage‐like lattices of ice trapping molecules of methane. Methane can also seep into the atmosphere through tapping of natural gas, while livestock digestion is another major source. “Understanding the relative roles of methane sources in the total atmospheric budget will help to frame the correct policy responses to controlling methane concentration rise”, commented Grant Allen, a specialist in the global methane budget at the University of Manchester, UK. “It will also help science predict future climate change under various IPCC scenarios with updated projections underpinned by new science”.

The issue is that people have to understand that fossil fuel and ecosystem carbon are two distinct carbon stocks.

Methane emissions can further be categorized into biogenic and thermogenic sources, arising, respectively, from organic and geological processes. Biogenic sources include lifestock farming, landfill refuge, and wetlands, which are largely “natural”. Similarly, thermogenic sources include coal mining, power stations, and oilrigs as well as natural geological seepage. A recent study calibrated the relative contributions, based on the fact that methane produced from biogenic sources contains less of the isotope carbon‐13 than methane from thermogenic sources, which are primarily associated with fossil fuel extraction and use 3. The authors found that biogenic sources contribute in the range of 55–70% to methane emissions and thermogenic sources 30–45%.

The recognition of the importance of natural carbon sinks in reducing the pace of climate change and the role of habitat conservation are both biogeochemical problems.

The study also determined that while fossil fuel methane emissions, including some natural geological seepage, were not increasing over time, it is still 60–110% greater than previous estimates. The message is that the fossil fuel industry still has potential to reduce anthropogenic climate change, even though methane emissions from natural gas as a fraction of production had declined from around 8% to 2% over the past three decades.

From biofilms to oceans

Given that the concentration of greenhouse gases will continue to increase for decades to come, an important role for biogeochemistry lies in understanding the role and function of global carbon sinks that could sequester some of the atmospheric CO₂. “The recognition of the importance of natural carbon sinks in reducing the pace of climate change and the role of habitat conservation are both biogeochemical problems”, Houlton explained. “Biogeochemistry has a long‐standing tradition of understanding the ultimate constraints on biological systems. Such constraints are generally input‐driven, meaning that the ways in which energy and biologically essential elements enter ecosystems determines the carbon cycle and ecosystem functioning”.

It raises more questions than answers though, commented Pieter Tans, who leads the Carbon Cycle Greenhouse Gases Group of the NOAA (National Oceanic and Atmospheric Administration) Earth System Research Laboratory in Boulder, Colorado. “The surprising thing is that the global net terrestrial sink seems to be keeping up with emissions”, he said. “We published a paper on that in 2012 and there is ongoing research with some reasonable hypotheses, but I would not say that we understand it 4”.

This research studies how surface infrastructures shuffle the key basic elements around the globe, notably the oceans, rivers, and streams that extend across the landscape into the soil and the phyllosphere, the part of plants above ground. It came as little surprise that coastal regions punch way above their size in terms of carbon and nutrient recycling, but it was less expected just what a big role lakes, rivers, and streams play. These are in effect the ecosystem arteries of the land, according to Tom Battin, Head of the Stream Biofilm and Ecosystem Research Laboratory at the EPFL (École polytechnique fédérale de Lausanne) in Switzerland. “It is only a few years since we realized how important inland waters are for global carbon fluxes”, he said. “This is close to a paradigm shift and has triggered massive research into the carbon fluxes in streams, rivers and lakes”.

It revealed that bacterial biofilms play a central role in this all‐important global network. “Biofilms dominate microbial life in streams and rivers, drive crucial ecosystem processes and contribute substantially to global biogeochemical fluxes”, Battin explained. “In turn, water flow and related deliveries of nutrients and organic matter to biofilms constitute major constraints on microbial life”. Battin therefore called for an integrated “omic” analysis that blends metagenomics, metatranscriptomics, and metaproteomics to unravel key functional capabilities of major bacterial taxa and map these onto interaction networks 5. This should also help to analyze the two‐way relationship between biofilms and the larger environment: On the one hand, climate and environmental change have a great impact on the biogeochemistry of stream ecosystems, but those changes mediated by biofilms in turn can have a significant feedback into those large‐scale processes.

Wetlands as carbon sinks

Recent work has also pinpointed the role of coastal wetlands along saltwater and freshwater systems, which again play a much greater role in global carbon fluxes than their surface area would suggest. According to one recent study, these wetlands have an area of < 2% of the oceans and yet contribute 50% to organic carbon storage 6. Coastal wetlands are frequently anoxic, being covered in water, which limits carbon decomposition and subsequent release in the underlying soils. These habitats are therefore a globally significant carbon storage system, which puts even greater emphasis on their preservation. This is especially the case for mangrove forests surrounded by seagrass meadows, which are one of the world's most productive ecosystems and have been recognized as global hot spots for soil carbon storage, which is twice as much as terrestrial soils.

Biofilms dominate microbial life in streams and rivers, drive crucial ecosystem processes and contribute substantially to global biogeochemical fluxes.

Biogeochemistry has clearly come a long way in terms of identifying key components and hot spots in the principal global nutrient and element cycles, with implications for all branches of environmental science as well as political actions to mitigate global climate change. It is also clear that while the fundamental principles are now quite well understood, the devil lies in the details yet to be explicated.