Patrick W. Keys

The rainforests of South America and Africa are considerably reliant on rainfall supplied by evaporation from the rainforests themselves through moisture recycling. Thus, rainforest loss in these areas can reduce evaporation as well as subsequent rainfall in the wider forested region. During dry periods, a dampened water cycle may potentially aggravate rainforest water stress. However, few studies have investigated dry‐period moisture recycling statistics within rainforests over longer records and consistently compared the rainforest moisture recycling in both continents. Here, we analyze dry‐season anomalies of moisture recycling for mean‐years and dry‐years, in the South‐American (Amazon) and African (Congo) rainforests over the years 1980–2023. We find that dry‐season rainfall reliance on rainforest moisture increases slightly, but statistically significantly, by 5% (from a mean annual of 21% to 22%) in the Amazon and up to 12% (from 24% to 27%) in the Congo. At the same time, a lower fraction of rainforest evaporation stays within the rainforest during the dry season: 21% less from the Amazon (from 41% to 32%) and 22% less from the Congo rainforest (from 32% to 25%), which further decreases with increasing dryness. In response, areas downwind of the rainforests gain a higher dependency on rainforest moisture during dry seasons, amplified during dry years. For example, rainforest‐sourced rainfall in Ecuador increases from annually to in the dry season. The findings highlight the critical role of rainforests in maintaining rainfall during periods when water is already scarce.

Rapid swings between extreme heat and precipitation events, often referred to as weather whiplash, have been increasingly observed around the world in a changing climate. Such sequential compound extreme events can threaten ecosystems and cause substantial socioeconomic losses due to short recovery time. Yet, historical trends in successive hot-wet extremes remain less understood, including how dry and humid heatwaves differently influence conditions for extreme precipitation. Using historic climate data, we examine how sequential hot-wet extremes have evolved in recent decades across the contiguous United States (CONUS). Given the region’s diverse climate, we compare the characteristics of dry and humid events using a locally defined extreme humidity threshold. We find that rapid transitions from heatwaves to extreme precipitation have become more frequent across the CONUS, with humid events occurring more often than dry types. Hotspot regions for dry events are concentrated in the Southwest and Southeast, where enhanced instability and moisture transport are observed, while humid events are dominant along the Gulf of Mexico and the East coast, with trends in increased moisture transport and reduced local instability. These findings strengthen our understanding of these climate extremes, and could support climate adaptation strategies and local resilience to weather whiplash.

Uncertainty in long-term climate outcomes arises not only from physical processes but also from societal responses to climate variability and change. Here we embed a range of temperature anomalies into an empirically-informed, coupled climate–social model to investigate how natural temperature variability, through its influence on societal decision-making, shapes global emissions trajectories. Using Monte Carlo ensembles spanning social, political, and technological parameters, we find that anomalous mid-century warmth strengthens public support for mitigation and accelerates the timing of net-zero emissions by several years, whereas anomalous cooling delays net-zero achievement. The response scales with the magnitude and persistence of variability with stronger and longer-lasting temperature anomalies producing larger shifts in mitigation timing. Across ensembles, high-variability climates are associated with higher emissions pathways and a greater likelihood of failing to reach net zero by 2100. Externally forced cooling events, such as volcanic eruptions, produce sharp impacts, delaying net zero by nearly a decade through temporary declines in public support for climate mitigation. These findings reveal that climate variability can cascade through perception, politics, and technology adoption to alter the pace of global mitigation, highlighting the sensitivity of emissions pathways to short-term climate signals and the role of societal interpretation of those signals.

Atmospheric moisture transport is an essential component of the Earth's water cycle, carrying water vapor thousands of miles and controlling which regions receive precipitation and which do not. Here, we employ atmospheric moisture tracking, applied to 35 years of reanalysis data to investigate decadal trends in atmospheric moisture transport for moisture precipitating in various watershed regions within the contiguous United States of America (CONUS). We find that across portions of CONUS water vapor is traveling farther over longer periods of time before precipitating, and less of it is coming from terrestrial sources. These findings confirm a trend that has been predicted by theory and models and have implications for future water resource governance.

In 2023, parts of eastern Colorado experienced their wettest three-month period (May–July) out of 129 years of record. This extreme precipitation led to flash flooding, road washouts, and significant property damage among Colorado communities along the Front Range including Denver, Boulder, and Fort Collins. Although the Front Range is the most populated region in Colorado, few studies have explored the evaporative origin of Front Range precipitation. To better anticipate and understand extreme precipitation events across the region, we focus our investigation on the evaporative origin of the extreme precipitation in May, June, and July (MJJ) of 2023 and how it compares to moisture sources during the previous two decades. This study uses the Water Accounting Model 2 Layers (WAM2layers) and hourly ERA5 reanalyses to quantify the evaporative sources of precipitation in Colorado’s Front Range during MJJ of 2023 and over the past 23 years (2000–2022). Our moisture source analysis reveals that for the Front Range region in May–July of 2023: (1) the three primary moisture sources are the Pacific Ocean, the western United States, and Colorado itself, contributing just over 66.2 % of total precipitation; (2) historically, those same regions dominate evaporative contributions in MJJ, but terrestrial contributions and local moisture recycling (i.e., precipitation that recently evaporated from within the Front Range) in May–July of 2023 accounted for a slightly greater proportion of precipitation than on average; (3) moisture sources in May–July 2023 were a statistical outlier in terms of the magnitude of moisture contributed to the Front Range; and (4) anomalous evaporative contributions in May–July 2023 originated from a broad range of regions, more consistent with the leading mode of moisture source variability, which reflects widespread anomalies from Colorado and the Pacific Ocean, rather than the second leading mode characterized by a north–south dipole in anomalous contributions from regions north and south of the Front Range. This research provides new insights into the origins of extreme rainfall in the summer of 2023 as well as the historical moisture sources of warm-season precipitation in the Front Range.

Afforestation efforts within tropical landscapes are continuing apace to achieve goals related to climate change mitigation, sustainability and conservation. Outcomes from these efforts are likely to be determined by future changes in climate and disturbance regimes. In Kenya, a tropical nation dominated by dryland ecosystems, the government is undertaking an ambitious ecosystem restoration effort via an afforestation program. Using L‐Range, an ecosystem model tuned to the conditions extant across Kenya, and the most recent downscaled climate projections we, explored the durability and wider impacts of these efforts. Conditioning the simulations on the achievement of 10% tree cover by 2030, we explored how woody vegetation, herb cover, and ecosystem productivity will respond in the near‐term (2050) under multiple Shared Socioeconomic Pathways and scenarios of fire frequency. Our simulations indicate that, under all scenarios, tree cover across Kenya will remain stable or show increasing trends in the near term. This will be accompanied by increases in overall woody vegetation, driven by shrub cover expansion and the contraction of herb cover. These impacts will be particularly pronounced in areas dominated by savannas and deciduous tree cover. Alongside these changes in vegetative cover, simulations indicate declines in net primary productivity and aboveground live biomass. Thus, we find that the persistence of existing and expanded Kenyan tree cover may not be impeded by climate change or disturbance; however, climate change and afforestation can act in concert to undermine achievement of Kenyan goals related to conservation and sustainability.

In 2023, parts of eastern Colorado experienced their wettest three-month period (May–July) out of 129 years of record. This extreme precipitation led to flash flooding, road washouts, and significant property damage among Colorado communities along the Front Range including Denver, Boulder, and Fort Collins. Although the Front Range is the most populated region in Colorado, few studies have explored the evaporative origin of Front Range precipitation. To better anticipate and understand extreme precipitation events across the region, we focus our investigation on the evaporative origin of the extreme precipitation in May, June, and July (MJJ) of 2023 and how it compares to moisture sources during the previous two decades. This study uses the Water Accounting Model 2 Layers (WAM2layers) and hourly ERA5 reanalyses to quantify the evaporative sources of precipitation in Colorado’s Front Range during MJJ of 2023 and over the past 23 years (2000–2022). Our moisture source analysis reveals that for the Front Range region in May–July of 2023: (1) the three primary moisture sources are the Pacific Ocean, the western United States, and Colorado itself, contributing just over 66.2 % of total precipitation; (2) historically, those same regions dominate evaporative contributions in MJJ, but terrestrial contributions and local moisture recycling (i.e., precipitation that recently evaporated from within the Front Range) in May–July of 2023 accounted for a slightly greater proportion of precipitation than on average; (3) moisture sources in May–July 2023 were a statistical outlier in terms of the magnitude of moisture contributed to the Front Range; and (4) anomalous evaporative contributions in May–July 2023 originated from a broad range of regions, more consistent with the leading mode of moisture source variability, which reflects widespread anomalies from Colorado and the Pacific Ocean, rather than the second leading mode characterized by a north–south dipole in anomalous contributions from regions north and south of the Front Range. This research provides new insights into the origins of extreme rainfall in the summer of 2023 as well as the historical moisture sources of warm-season precipitation in the Front Range.

Uncertainty in long-term climate outcomes arises not only from physical processes but also from societal responses to climate variability and change. Here we embed a range of temperature anomalies into an empirically-informed, coupled climate–social model to investigate how natural temperature variability shapes global emissions trajectories. Using Monte Carlo ensembles spanning social, political, and technological parameters, we find that anomalous mid-century warmth accelerates the timing of net-zero emissions by several years, while anomalous cooling delays net-zero achievement. The response scales with the magnitude and persistence of variability with stronger and longer-lasting temperature anomalies producing larger shifts in mitigation timing. Across ensembles, high-variability climates are associated with higher emissions pathways and a greater likelihood of failing to reach net zero by 2100. Externally forced cooling events, such as volcanic eruptions, produce sharp impacts, delaying net zero by nearly a decade through temporary declines in public support for climate mitigation. These findings reveal that climate variability can cascade through perception, politics, and technology adoption to alter the pace of global mitigation, highlighting the sensitivity of emissions pathways to short-term climate signals.

In 2023, parts of eastern Colorado experienced their wettest three-month period (May–July) out of 129 years of record. This extreme precipitation led to flash flooding, road washouts, and significant property damage among Colorado communities along the Front Range including Denver, Boulder, and Fort Collins. Although the Front Range is the most populated region in Colorado, few studies have explored the evaporative origin of Front Range precipitation. To better anticipate and understand extreme precipitation events across the region, we focus our investigation on the evaporative origin of the extreme precipitation in May, June, and July (MJJ) of 2023 and how it compares to moisture sources during the previous two decades. This study uses the Water Accounting Model 2 Layers (WAM2layers) and hourly ERA5 reanalyses to quantify the evaporative sources of precipitation in Colorado’s Front Range during MJJ of 2023 and over the past 23 years (2000–2022). Our moisture source analysis reveals that for the Front Range region in May–July of 2023: (1) the three primary moisture sources are the Pacific Ocean, the western United States, and Colorado itself, contributing just over 66.2 % of total precipitation; (2) historically, those same regions dominate evaporative contributions in MJJ, but terrestrial contributions and local moisture recycling (i.e., precipitation that recently evaporated from within the Front Range) in May–July of 2023 accounted for a slightly greater proportion of precipitation than on average; (3) moisture sources in May–July 2023 were a statistical outlier in terms of the magnitude of moisture contributed to the Front Range; and (4) anomalous evaporative contributions in May–July 2023 originated from a broad range of regions, more consistent with the leading mode of moisture source variability, which reflects widespread anomalies from Colorado and the Pacific Ocean, rather than the second leading mode characterized by a north–south dipole in anomalous contributions from regions north and south of the Front Range. This research provides new insights into the origins of extreme rainfall in the summer of 2023 as well as the historical moisture sources of warm-season precipitation in the Front Range.

Atmospheric moisture transport is an essential component of the Earth’s water cycle, carrying water vapor thousands of miles and controlling which regions receive precipitation and which do not. Here, we employ atmospheric moisture tracking, applied to 35 years of reanalysis data to investigate decadal trends in atmospheric moisture transport for moisture precipitating in various watershed regions within the contiguous United States of America (CONUS). We find that across portions of CONUS water vapor is traveling farther over longer periods of time before precipitating, and less of it is coming from terrestrial sources. These findings confirm a trend that has been predicted by theory and models and have implications for future water resource governance.