Terrestrial Systems' Impact on and Response to Climate Change

Terrestrial systems are both a source and a sink of carbon emissions and play a fundamental role in regulating climate change. To deliver on the Paris Agreement goal of limiting warming to 1.5°C or 2°C, clarifying terrestrial systems’ mitigation potential and their response to climate change is critical. The goal of my dissertation was to investigate terrestrial systems’ potential contribution to the Paris Agreement mitigation pathway (Chapter 2), the impact of climate change on forest sequestration potential (Chapter 3), and the effect of climate on tropical forest litter carbon turnover and consequent impacts on the tropical forest sink (Chapter 4). The meta-analysis of land-based mitigation and roadmap to 2050 developed in Chapter 2 showed that deploying measures in agriculture, forestry, wetlands, and bioenergy could feasibly and sustainably contribute ~30% (14-15 GtCO2e yr-1) of the global mitigation needed in 2050 to deliver on the 1.5°C target. Land-based emissions would need to decline by ~50% per decade (85% gross reductions by 2050) and carbon removals would need to increase ten-fold by 2050 to make the land sector net zero emissions by 2050. Both 2°C and 1.5ºC temperature targets require steep emission reductions from tropical deforestation, yet the 1.5ºC goal will require earlier and deeper reductions in agricultural and demand-side emissions, and enhanced carbon removals from reforestation, soil carbon sequestration, agroforestry and forest management.

Land-based measures that enhance carbon removals are likely to be affected by future climate change. However, very few studies that estimate land-based sequestration potential consider climate impacts. My study on biophysical sequestration potential for afforestation, reforestation and forest enhancement (A/R/E) under two climate futures in Chapter 3 is one of the first to do so. A/R/E has the potential to sequester 3.8-7.3 GtCO2 yr-1 in 2050 depending on future agricultural expansion, with ~45% from afforestation, ~33% from reforestation, and ~21% from forest enhancement. High levels of future agricultural expansion (+650 Mha) not only reduces the A/R/E sequestration potential by 41%, it also substantially reduces (by 62%) the natural capacity of land to act as a carbon sink. Reforestation and forest enhancement in the tropics and sub-tropics have higher mitigation densities than afforestation, higher potential to deliver multiple benefits, and are most aligned with countries’ restoration pledges. In a 4°C climate future (7.0 W/m2 forcing), sequestration potential is ~20% greater, with 15-30% higher gains in the tropics compared to temperate and boreal regions. Productivity increases outweighed carbon losses from ecosystem respiration and fire, largely due to CO2 fertilization. However, the strength of carbon–concentration and carbon–climate feedbacks over land is highly uncertain. Responses in tropical forest soils in particular, including impacts on decomposition and carbon turnover, are poorly represented in models and are a critical source of uncertainty.

In Chapter 4, I evaluated the effects of sustained 4°C warming on in-situ litter decomposition in a tropical forest and then compared the experimental field results to the earth system model results from Chapter 3. I found that warming reduced mass loss by an average of 7% across four different substrates. Warming decreased litter moisture by an average of 36%, relative humidity by 4%, and soil moisture by 1.2%, which appear to have limited microbial activity and decomposition. However, the effect of warming on reduced mass loss varied among the substrates, with a stronger response in lower quality (higher C:N) substrates. These results suggest that temperature increases with concomitant drying could significantly slow carbon and nutrient turnover from lower quality litter to soil. In the model experiment, we also found reduced litter carbon turnover rates in tropical forests that experienced drying, but with lower sensitivity. Although litter carbon turnover decreased across most dry tropical forests with reduced precipitation, it only decreased in wet tropical forests that experienced higher levels of drying than occurred in our field experiment. The Chapter 4 findings suggest carbon turnover with future climate change could depend more strongly on moisture regimes in wet tropical forests than currently captured in models.