Xun Jiang

In this study, we examine the precipitation, vegetation index, photosynthesis, net ecosystem exchange (NEE), atmospheric CO ₂ concentrations, and horizontal and vertical transports over the Sahara Desert. The precipitation during August–October is over 17 times higher than in January–March, which is driven by Intertropical Convergence Zone migration, stronger African Monsoon, and increased rising motion over the Sahara. Photosynthetic activity, monitored by Solar‐Induced Fluorescence (SIF), is twice as high in August–October than in January–March, highlighting the impact of rainfall on the plant's growth. Increased photosynthetic activities (high SIF) and enhanced CO ₂ uptake by the biosphere (negative NEE) will remove more CO ₂ from the atmosphere, resulting in atmospheric CO ₂ levels being ∼4 ppm lower during August–October than January–March. Lower CO ₂ levels in August–October are also captured by the CarbonTracker model. We further analyze correlations among detrended timeseries of one‐month‐lead precipitation, normalized difference vegetation index, and NEE averaged over the Sahara Desert during the wet season. The results show significant correlations, with higher precipitation associated with increased vegetation coverage and stronger CO ₂ uptake. A long‐term analysis reveals increasing trends in both precipitation and vegetation index and a decreasing trend in NEE during the wet season. These findings suggest that the Sahara Desert is becoming greener and evolving into a stronger carbon sink, as increased vegetation absorbing more CO ₂ from the atmosphere.

Reply to Gao et al. and Xu: Greening the Taklamakan: Human efforts to convert desert into a carbon sink

The Taklamakan Desert, one of the world’s largest and driest deserts, has traditionally been considered a biological void. Here, we demonstrate that large-scale ecological restoration is transforming this hyperarid environment into a carbon sink. By analyzing satellite and ground-based data, we find strong seasonal dynamics: During the wet season (Jul to Sep), precipitation increases to 16.3 mm/mo, enhancing vegetation coverage and photosynthetic activity and drawing down atmospheric CO ₂ by approximately three parts per million (ppm) relative to the dry-season levels. Long-term trends reveal significant increases in vegetation cover (6.8 × 10 ⁻⁴ /y) and photosynthetic activity (6.1 × 10 ⁻³ W/m ² /sr/µm/y), accompanied by a strengthening net CO ₂ uptake (NEE trend: −5.2 × 10 ⁻¹² kg/m ² /s/y). These changes are spatially concentrated along the desert margins and their timing aligns with implementation of China’s Three-North Shelterbelt Program. Our results provide the direct evidence that human-led intervention can effectively enhance carbon sequestration in even the most extreme arid landscapes, demonstrating the potential to transform a desert into a carbon sink and halt desertification. This underscores the critical role of dryland restoration in global carbon management strategies and highlights the Taklamakan Desert as a model for climate change mitigation through nature-based solutions and ecological engineering.

Wildfires have broad impacts on the atmosphere, ecology, and society. This study leverages satellite data and chemistry‐transport models to analyze the impact of wildfires on trace gases in California during the August‐October periods of 2018, 2019, and 2020. During these months, Southern California experiences minimal precipitation, leading to a high Vapor Pressure Deficit, which results in decreased photosynthetic activities. This reduction, combined with increased biomass burning, causes a rise in CO₂ concentrations. Increased CO and CH₄ levels are also seen in TROPOMI retrievals tied to the increase in biomass burning. The CarbonTracker model captures these elevated CO₂ concentrations, though with a reduced amplitude of increased CO₂. Similarly, the GEOS‐Chem model successfully simulates high CO levels but underestimates the observed enhancements. These findings will improve the understanding of fire's influence on trace gases and refine future numerical models on surface emissions and transport.

Over the tropical Indo‐Pacific, we explore the variations of Solar‐Induced Fluorescence (SIF), CO₂, and precipitation during El Niño Southern Oscillation (ENSO) events verse other times. Based on various data sets, we analyze the time series of deseasonalized precipitation, vertical velocity, SIF, and CO₂ across the Indo‐Pacific region. Our analysis reveals that there is less precipitation, more sinking air, lower SIF, and higher carbon dioxide over the Indo‐Pacific region during the El Niño events compared to other times. We also explore the spatial patterns of different variables to gain a deeper understanding of their relationships. The spatial analysis suggests that less precipitation resulting from predominant sinking air not only leads to lower SIF values by reducing photosynthetic activities but also triggers more biomass burning, which contribute to an increase in atmospheric CO₂ levels across most areas of the Indo‐Pacific region. The comparative study between observational analyses and numerical simulation by the NOAA CarbonTracker model indicates that the model can simulate the increase of atmospheric column CO₂ during the El Niño event, although there are some difficulties in capturing the correct spatial pattern. The results of this study can significantly enhance our understanding of the spatial‐temporal variabilities of atmospheric CO₂ concentration and the observational characteristics can be used to enhance models. Furthermore, the complex interactions between ENSO and other variables revealed in this study improve our understanding of how ENSO influences the biosphere and carbon cycle.

Mid‐tropospheric Carbonyl sulfide (OCS) retrievals from the Tropospheric Emission Spectrometer (TES) are utilized to study OCS distributions during the dry/wet seasons over the Amazon rainforest. TES OCS retrievals reveal positive OCS anomalies (∼16 ppt) over the central and southern parts of the Amazon during August–October (dry season) compared to January–March (wet season). There is less OCS taken up by vegetation and soil and more OCS released from biomass burning during the dry season, which causes an increase in OCS concentrations. Strong sinking air during the dry season also helps to trap OCS and this contributes to positive OCS anomalies. MOZART‐4 model captures positive OCS anomalies over the central and southern regions of the Amazon and negative OCS anomalies over the northern part of the Amazon, which are similar to those from TES mid‐tropospheric OCS retrievals. Our studies can help us better understand OCS variations and photosynthetic activities.

The Congo basin, with an area of about 3.7 million km² in the tropical region, contains the second largest rainforest and is considered as a carbon sink for the atmosphere. Here, using Orbiting Carbon Observatory‐2 satellite observations, we show that the atmospheric CO₂ over the Congo basin is ∼2 ppm higher than the regional background during June–August, which is primarily due to biomass burning and significantly reduced photosynthetic activities during the dry season. The contribution from the biomass burning is larger than that from the biosphere during June–August. Current budget estimations suggest emissions from biomass burning during the dry season alone account for ∼72% of the Congo basin annual biomass burning emissions and are ∼40% more than the largest terrestrial uptake in the same region during January–March (wet season). Therefore, better seasonal fire management in this region is an important strategy for achieving timely reductions in global carbon emissions as set by international agreements.

Previous studies suggested that the Amazon, the largest rainforest on Earth, changes from a CO₂ sink to a CO₂ source during the dry/fire season. However, the biospheric contributions to atmospheric CO₂ are not well understood during the two main seasons, the dry/fire season and the wet season. In this article, we utilize Orbiting Carbon Observatory 2 (OCO‐2) Solar‐Induced Fluorescence (SIF) to explore photosynthetic activity during the different seasons. The spatiotemporal variability of OCO‐2 SIF, OCO‐2 CO₂, precipitation, and burned area are investigated over the Amazon from September 2014 to December 2019. Averaging over the entire Amazon region, we found a positive temporal correlation (0.94) between OCO‐2 SIF and Global Precipitation Climatology Project precipitation and a negative temporal correlation (−0.64) between OCO‐2 SIF and OCO‐2 CO₂, consistent with the fact that precipitation enhances photosynthesis, which results in higher values for SIF and rate of removal of CO₂ from the atmosphere above the Amazon region. We also observed seasonality in the spatial variability of these variables within the Amazon region. During the dry/fire (August–October) season, low SIF values, low precipitation, high vapor pressure deficit (VPD), large burned areas, and high atmospheric CO₂ are mainly found over the southern Amazon region. In contrast, during the wet season (January–March), high SIF values, high precipitation, low VPD, smaller burned areas, and low CO₂ are found over both the central and southern Amazon regions. The seasonal difference in SIF suggests that photosynthetic activity is reduced during the dry/fire season relative to the wet season as a result of low precipitation and high VPD, especially over the southern Amazon region, which will contribute to more CO₂ in the atmosphere during the dry/fire season.

Radiant energies of planets and moons are of wide interest in the fields of geoscience and planetary science. Based on long‐term multiinstrument observations from the Cassini spacecraft, we provide here the first observational study of Titan's global radiant energy budget and its seasonal variations. Our results show that Titan's radiant energy budget is not balanced over the Cassini era (2004–2017) with the absorbed solar energy (1.208 ± 0.008) × 10²³ J larger than the emitted thermal energy (1.174 ± 0.005) × 10²³ J. The energy imbalance is 2.9 ± 0.8% of the emitted thermal energy. Titan's global radiant energy budget is not balanced either at the timescales of Earth's years and Titan's seasons. In particular, the energy imbalance can be beyond 10% of the emitted thermal energy at the timescale of an Earth year. The energy imbalance revealed in this study has important impacts on Titan, which should be examined further by theories and models.

Amazon rainforest fires have significant environmental and societal impacts, but the mechanism and impact of the fires on the regional and global carbon cycles have not been fully understood. Over the rainforest, less precipitation, higher surface temperature, and enhanced mid‐tropospheric sinking air over the eastern part of the Amazon characterized the fire/dry season. These meteorological conditions will facilitate more fires in the Amazon rainforest. Using the Orbiting Carbon Observatory‐2 column CO₂, we notice that there are ∼2 ppm more CO₂ over the Amazon compared with the surrounding area during the fire season. The higher concentrations of atmospheric CO₂ are related to the surface biomass burning, enhanced sinking air over the eastern part of the Amazon, and surface winds. Results from this study can help us better understand the carbon sources and sinks over the Amazon during the fire/dry season.