最新の研究成果 How can the global carbon cycle respond to CO₂ and non-CO₂ mitigation?

Our new study explores how reducing non-CO₂ greenhouse gases—like methane and nitrous oxide—alongside CO₂ reductions can facilitate more effective climate change mitigation. While both CO₂ and non-CO₂ greenhouse gases drive global warming, they influence “natural” carbon absorption by land and ocean—known as “carbon sinks”—in different ways. This research highlights that targeting both CO₂ and non-CO₂ gases together can better protect carbon sinks on land and in the oceans for more effective and sustainable climate policy.

The study used advanced Earth system model simulations to explore how carbon dioxide, methane, and nitrous oxide affect global warming and carbon sinks under a set of experiments with both increasing and decreasing concentrations. The carbon sinks act as natural buffers, absorbing and storing CO₂, which helps slow the rate of climate change. However, as temperatures rise, the effectiveness of land and ocean carbon sinks decreases, leading to acceleration of global warming.

While both CO₂ and non-CO₂ greenhouse gases trap heat, they affect natural carbon sinks on land and in the oceans in different ways. CO₂ contributes to climate change by trapping heat. But it also enhances the carbon sinks: on land, it boosts plant growth—a process known as "CO₂ fertilisation"—and in the oceans, it increases carbon absorption through the "solubility pump". This means that the increase in CO₂ concentration has both warming and carbon sink-enhancing effects (Figure 1a). When the CO₂ concentration is reduced, both warming and the sink-enhancing effects are diminished: plant growth stimulated by CO₂ fertilisation decreases on land, and the ocean's ability to absorb carbon through the solubility pump weakens (Figure 1b).

Non-CO₂ greenhouse gases such as methane and nitrous oxide also trap heat, but obviously these gases do not enhance carbon sinks through fertilisation effect or solubility pump (Figure 1c). When concentrations of these gases are reduced, there is a cooling effect without direct impacts on the ability of the natural environment to absorb atmospheric carbon dioxide through fertilisation or the solubility pump (Figure 1d). This distinction makes reducing concentrations in non-CO₂ gases a valuable complement to the CO₂ mitigation efforts. Reducing methane concentrations, for example, can immediately lower the rate of warming because methane has a strong but short-lived warming effect. The combined mitigation that addresses both types of gases has the potential to deliver faster and greater cooling than tackling CO₂ alone.

At the core of these findings the study introduces an analytical framework that enables better understanding of the different ways greenhouse gases interact with the carbon cycle. The framework identifies three key feedback mechanisms that link the changes in CO₂ and non-CO₂ greenhouse gases and carbon sinks: (1) the effect of changes in CO₂ concentration on carbon sinks through CO₂ fertilisation effects and the solubility pump, (2) the effect of temperature changes on carbon sinks, and (3) a “combined effect” (Figure 2). The latter combined effect has the most complex representation, where changes in CO₂ concentrations and temperatures interact to influence carbon sinks. However, this combined effect can significantly influence the global carbon sink, especially over the ocean (see Figure 2b and c, where the effects of temperature changes and combined effect on the carbon sink are shown).

The findings underscore the need for multifaceted climate action strategies that combine CO₂ and non-CO₂ mitigation. This approach enables scientists and policymakers to strengthen natural climate sinks and work toward a more sustainable future.