Why Nature? Why Now?

How nature is key to achieving a 1.5˚C world

1. Climate change: greenhouse gas emissions, sources and sinks

Human activity has increased the release of greenhouse gases (GHGs) into the atmosphere

GHGs are the gaseous constituents that trap heat in the atmosphere. They are released through natural processes (e.g. decomposition of biomass) and as a result of human activity (e.g. the burning of fossil fuels). Some gases are naturally occurring (e.g. carbon dioxide) while others are human-made (e.g. the halocarbons). Carbon dioxide (CO2) is the largest single contributor to climate change. The United Nations Framework Convention on Climate Change covers the following GHGs:

Carbon dioxide


CO2 is naturally occurring but is also a by-product of burning fossil fuels, of burning biomass, of land-use changes and of industrial processes.



CH4 is the major component of natural gas and it is associated with all hydrocarbon fuels. Significant emissions also occur as a result of animal husbandry, waste management and agriculture.

Nitrous oxide


The main anthropogenic source of N2O is agriculture, in addition to sewage treatment, fossil fuel combustion, and chemical industrial processes. N2O is also produced naturally e.g. through microbial action in wet tropical forests.

Fluorinated gases


F-gases include sulphur hexafluoride (man-made chemical primarily used in electrical transmission and distribution systems, and in electronics), hydrofluorocarbons and perfluorocarbons (alternatives to ozone depleting substances, these by-products of industrial processes are powerful GHGs).

The GWP allows comparisons of the global warming impacts of different gases over specific timeframes. CO2 is the reference gas and so the GWP is 1.

Increasing concentrations of GHGs in the atmosphere have caused a warming of the Earth's mean surface temperature. This is referred to as the greenhouse effect.

Human activities release
greenhouse gases

Carbon dioxide


CFCs & Haloalkane

Nitrous oxide

The three main systems capable of storing carbon and nitrogen, known as “stocks” or “pools”, include the land ecosystems, the ocean and the Earth’s crust.

Carbon and nitrogen not stored in these pools resides in the atmosphere as a component of greenhouse gases.

Land ecosystems (such as forests and peatlands): Plants absorb carbon through photosynthesis. The carbon they capture is stored in vegetation or integrated into soils when plants die. The breakdown of plant material and soil by microorganisms leads to emissions.

The Earth’s deep mantle sequesters carbon through sedimentation and other geological formations, on geological timescales (many millennia). Carbon is released into the atmosphere through the extraction and combustion of fossil fuels.

Atmospheric CO2 dissolves into the ocean, and phytoplankton also sequester carbon by photosynthesis, while deep cold waters absorb carbon.

What is released or cannot be stored by other carbon stocks accumulates into the atmosphere.

Whether a stock is considered a "sink" or a "source" of greenhouse gases depends on the net flux of 1) emissions out of the stock and into the atmosphere and 2) removals from the atmosphere and into the stock.

Carbon inflow /
atmospheric removal
Carbon outflow /
Net (in/out) flux

Carbon sinks are the carbon pools capable of sequestering more carbon than they emit. They include the ocean and the land biosphere.

Carbon sources are those systems that emit more CO2 than they sequester over a period of time.

Carbon sinks are the carbon pools capable of sequestering more carbon than they emit. They include the ocean and the land biosphere.

Carbon sources are those systems that emit more CO2 than they sequester over a period of time.

For example, forests are the largest terrestrial sink - globally, their net removal of carbon is equivalent to 5.7 billion metric tonnes of carbon dioxide (GtCO2) a year. This represents 45% of carbon dioxide sequestration from the land sink.

But disturbances of land, ocean and geological stocks can result in net emissions of GHGs into the atmosphere, reducing the size of the global sinks.

The California wildfires in 2020 released more than 91 million metric tonnes of carbon dioxide into the atmosphere, 25% more than California’s annual emissions from fossil fuels. A large portion of these emissions will be recovered over coming centuries by vegetation regrowth; however, the increasing frequency of fire disturbance raises the possibility of long-term losses of forest carbon stocks to the atmosphere.

Forests, such as the Amazon or Russia’s boreal forests, are exposed to tipping points and Earth system feedback loops which could see them turn into net sources of carbon. The more the climate warms, the more likely these accelerating feedbacks and tipping points become.

The increasing frequency of regional disturbances such as fire can diminish regional sinks or trigger those sinks to become sources of GHGs. The more widespread these regional changes, the greater influence on the global GHG sinks.

This is already happening in forest areas across the tropical belt…

This map shows the net carbon sinks (green) and sources (red) from forests across the period 2001-19 (MtCO2e). The largest sinks are found in tropical forests. The largest sources are found in disturbed tropical forests.

Net annual forest-related greenhouse gases fluxes

2. Stock-take: the flow of greenhouse gas emissions into and out of the atmosphere today

In the case of CO2, human activity resulted in an average of 50.6 billion tonnes of gross anthropogenic CO2 emissions a year over the period 2010 to 2019.

Which includes 34.4 billion tonnes of CO2 emissions from fossil fuels and cement.

And 16.1 billion tonnes of CO2 emissions from human activities on land, including those leading to land-use change and forestry (often referred to as Land Use and Land Use Change and Forestry or LULUCF emissions).

Human activities on land can also result in atmospheric removals, for example through reforestation, afforestation or switching to regenerative agricultural practices. Over the same period, these human activities resulted in the removal of 10.6 billion tonnes of CO2 each year (on average).

A further 12.5 billion tonnes of CO2 were removed by the natural terrestrial sink (i.e. through natural processes not related to human activity).

And 9.2 billion tonnes of CO2 were removed by the natural ocean sink.

18.7 billion tonnes of CO2 remained in the atmosphere.

In summary, we are emitting more CO2 than can be removed by Earth’s systems…

And the story is similar for other greenhouse gases such as methane…

… and nitrous oxide.

Fossil fuel combustion and oxidation from all energy and industrial processes, also including cement production and carbonation.