Low-intensity fires mitigate the risk of high-intensity wildfires in California's forests.

The increasing frequency of severe wildfires demands a shift in landscape management to mitigate their consequences. The role of managed, low-intensity fire as a driver of beneficial fuel treatment in fire-adapted ecosystems has drawn interest in both scientific and policy venues. Using a synthetic control approach to analyze 20 years of satellite-based fire activity data across 124,186 square kilometers of forests in California, we provide evidence that low-intensity fires substantially reduce the risk of future high-intensity fires. In conifer forests, the risk of high-intensity fire is reduced by 64.0% [95% confidence interval (CI): 41.2 to 77.9%] in areas recently burned at low intensity relative to comparable unburned areas, and protective effects last for at least 6 years (lower bound of one-sided 95% CI: 6 years). These findings support a policy transition from fire suppression to restoration, through increased use of prescribed fire, cultural burning, and managed wildfire, of a presuppression and precolonial fire regime in California.

Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing.

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO(2), p(co(2))). Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (∼12-5 Myr ago), an interval with p(co(2)) values of only 200-350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low p(co(2)) ∼12-5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ∼13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between p(co(2)), sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.

Permanent El Niño-like conditions during the Pliocene warm period.

During the warm early Pliocene (approximately 4.5 to 3.0 million years ago), the most recent interval with a climate warmer than today, the eastern Pacific thermocline was deep and the average west-to-east sea surface temperature difference across the equatorial Pacific was only 1.5 +/- 0.9 degrees C, much like it is during a modern El Niño event. Thus, the modern strong sea surface temperature gradient across the equatorial Pacific is not a stable and permanent feature. Sustained El Niño-like conditions, including relatively weak zonal atmospheric (Walker) circulation, could be a consequence of, and play an important role in determining, global warmth.

Regional climate shifts caused by gradual global cooling in the Pliocene epoch.

The Earth's climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 degrees C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation approximately 2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

A transient rise in tropical sea surface temperature during the Paleocene-Eocene thermal maximum.

The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) (8 degrees to 10 degrees C) and in bottom water temperatures (4 degrees to 5 degrees C). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a 4 degrees to 5 degrees C rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2 to levels three to four times as high as those estimated for the late Paleocene.