Major changes in global rainfall patterns accompanied a northward shift of Earth's thermal equator at the onset of an abrupt climate change 14.6 kya. This northward pull of Earth's wind and rain ...belts stemmed from disintegration of North Atlantic winter sea ice cover, which steepened the interhemispheric meridional temperature gradient. A southward migration of Earth's thermal equator may have accompanied the more recent Medieval Warm to Little Ice Age climate transition in the Northern Hemisphere. As fossil fuel CO2 warms the planet, the continents of the Northern Hemisphere are expected to warm faster than the Southern Hemisphere oceans. Therefore, we predict that a northward shift of Earth's thermal equator, initiated by an increased interhemispheric temperature contrast, may well produce hydrologic changes similar to those that occurred during past Northern Hemisphere warm periods. If so, the American West, the Middle East, and southern Amazonia will become drier, and monsoonal Asia, Venezuela, and equatorial Africa will become wetter. Additional paleoclimate data should be acquired and model simulations should be conducted to evaluate the reliability of this analog.
We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the ...Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.
A number of key paleoclimate records in the Southern Hemisphere midlatitudes exhibit climate changes synchronous with abrupt climate changes in the North Atlantic. We advance a hypothesis – argued ...from consideration of model evidence, observational climate diagnostics, and atmospheric dynamics – that attributes said climate changes in the Southern Hemisphere to a modulation in the strength of the South Pacific Split Jet, a pronounced zonally asymmetric feature of the wintertime Southern Hemisphere westerlies. North Atlantic cooling is associated with a weaker Split Jet, characterized by weaker South Pacific subtropical and subpolar jets and a strengthened midlatitude jet. It leads to climate impacts over the South Pacific sector that coincides with regions with observed paleoclimate changes timed to the North Atlantic. These circulation changes are envisioned to operate in addition to the climate impacts resulting from the oceanic bipolar seesaw.
A proposed global atmospheric teleconnection links North Atlantic cooling to the weakening of the Split Jet. North Atlantic cooling induces a southward shift of the marine Intertropical Convergence Zone and weakening of the Asian monsoon. The resulting Hadley circulation change weakens the wintertime South Pacific subtropical jet, and which in turn leads to a weaker South Pacific Split Jet. A weaker Split Jet leads to a southward shift of the zero wind-stress curl line, implying a shift in the same sense for the South Pacific subtropical front. Over land, it leads to winter warming over New Zealand, winter cooling over subtropical South America, drying over Western Patagonia, and winter warming and wetting of southernmost Patagonia. Our hypothesis also predicts reduced storminess over West Antarctica. Similar changes but of opposite sign occur in the Northern Hemisphere, where a stronger wintertime North Pacific subtropical jet increases precipitation over the Western United States.
•We hypothesize the South Pacific Split Jet weakens during North Atlantic stadials.•An atmospheric teleconnection connects the North Atlantic to SH westerlies.•Connection works via ITCZ shift changing Hadley circulation and subtropical jets.•Existing South Pacific paleoproxy evidence appears consistent with hypothesis.•Hypothesis predict changes to West Antarctica timed to North Atlantic abrupt events.
Lake Estancia's transition from a Big Dry episode during the first half of the Mystery Interval to a Big Wet episode during the second half has equivalents in records from across the planet. At the ...time of this transition, Chinese monsoons experienced pronounced weakening, closed-basin lakes in both the Great Basin of the western United States and in the southern Altiplano of South America underwent a major expansion, mountain glaciers in Southern Hemisphere middle latitudes had retreated, and the rates of increase of CO2 and of δ18O in Antarctic ice underwent a decrease. Finally, the precipitous drop in dust rain over Antarctica and the Southern Ocean terminated as did a similar drop in the 13C to 12C ratio in atmospheric CO2. These changes are consistent with a southward shift of the thermal equator. The cause of such a shift is thought to be an expansion of sea ice caused by a shutdown in deep water production in the northern Atlantic. This creates a dilemma because a similar southward shift is an expected consequence of the Heinrich event #1 which initiated the Mystery Interval.
► A pronounced change occurred at the midpoint of the Mystery Interval. ► It appears in records from ice cores, stalagmites, pluvial lakes and river runoff. ► But it is absent in the Greenland dust record and in the decline of atmospheric 14C.
Mountain glaciers are highly sensitive to climate change. However, the extent to which glaciers capture regional to hemisphere‐scale atmospheric processes remains uncertain, hindering paleoclimatic ...interpretations derived from moraine‐based glacier reconstructions. Here, we evaluate how mid‐latitude glacier systems monitor climate by comparing climate reanalysis products with glacier annual equilibrium line altitude (ELA) elevations from the antipodal Southern Alps of New Zealand and European Alps. We find significant regional and hemispheric correlations between glacier annual ELA and summer tropospheric temperatures. Annual ELA also exhibit positive correlations with the latitude of the westerly jets in both hemispheres. These results indicate that westerly wind‐belt latitude modulates the proportion of cold versus warm air masses influencing these glacier systems. These results highlight the sensitivity of mid‐latitude glaciers to atmospheric temperatures and circulation, with implications for interpreting moraine‐based paleoclimate reconstructions. Combined impacts of ongoing tropospheric warming and poleward‐shifting westerlies will likely accelerate recession of mid‐latitude glaciers.
Plain Language Summary
Mountain glaciers respond to climate change by gaining mass when the climate cools and losing mass when the climate warms. However, the extent to which these glacial fluctuations are reflective of local, regional, and hemispheric climate variations is less clear, hindering climatic interpretation of paleo‐glacier reconstructions developed from glacial landforms. This study evaluates the climatic footprint monitored by antipodal mid‐latitude glacier populations by comparing gridded reconstructions of global temperature and wind changes with glacier annual snowline elevations in the Southern Alps of New Zealand and annual equilibrium line altitude elevations in the European Alps. Our results indicate that (a) these glacier systems co‐vary with atmospheric temperatures on regional and even hemispheric scales throughout all levels of the troposphere, and (b) the latitudes of the westerly wind belts are important for regulating the proportion of cold versus warm air masses influencing glacier mass‐balance. Altogether, our results indicate that mid‐latitude mountain glacier fluctuations reflect temperature changes integrated over large regions of the atmosphere. With ongoing climate change, the combination of global atmospheric warming and poleward‐shifting westerlies is likely to accelerate recession of mid‐latitude glaciers in both hemispheres.
Key Points
Mid‐latitude glacier annual equilibrium line altitude corresponds to broad regions of atmospheric temperature
Mid‐latitude glacier annual equilibrium line altitude is sensitive to latitudinal shifts of the mid‐latitude westerlies
The influence of the westerlies on glaciers has important implications for interpreting past and predicting future climate change
We examine the timing and magnitude of the last glacial maximum (LGM) and the last glacial termination (LGT) in northwestern Patagonia, situated in the middle latitudes of South America. Our data ...indicate that the main phase of the LGT began with abrupt warm pulses at 17,800 and 17,100 cal yrs BP, accompanied by rapid establishment of evergreen temperate rainforests and extensive deglaciation of the Andes within 1000 years. This response shows that South American middle-latitude temperatures had approached average interglacial values by 16,800 cal yrs BP. The temperature rise in northwestern Patagonia coincides with the beginning of major warming and glacier recession in the Southern Alps of New Zealand at southern mid-latitudes on the opposite side of the Pacific Ocean. From this correspondence, the warming that began at 17,800 cal yrs BP appears to have been widespread in middle latitudes of the Southern Hemisphere, accounting for at least 75% of the total temperature recovery from the LGM to the Holocene. Moreover, this warming pulse is coeval with the first half of the Heinrich Stadial 1 (HS1) in the North Atlantic region. HS1 featured a decline of North Atlantic meridional overturning circulation, a southward shift of the westerly wind belt in both hemispheres and of the Intertropical Convergence Zone, as well as a weakening of the Asian monsoon. Along with the initiating trigger, identifying the mechanisms whereby these opposing climate signals in the two polar hemispheres interacted —whether through an oceanic or an atmospheric bipolar seesaw, or both— lies at the heart of understanding the LGT.
•The NW sector of the Patagonian ice sheet expanded 5 times between 17,700 and 33,600 cal yrs BP.•Warming at 17,800 cal yrs BP drove abrupt expansion of rainforests and withdrawal of Andean glaciers.•The 17,800 cal yrs BP event was a decisive trigger for the Last Glacial Termination (LGT).•Holocene-like conditions were achieved within 1000 years after the onset of the LGT.•Northward-shifted westerlies during the LGM migrated back south at the onset of the LGT.
A likely consequence of global warming will be the redistribution of Earth's rain belts, affecting water availability for many of Earth's inhabitants. We consider three ways in which planetary ...warming might influence the global distribution of precipitation. The first possibility is that rainfall in the tropics will increase and that the subtropics and mid-latitudes will become more arid. A second possibility is that Earth's thermal equator, around which the planet's rain belts and dry zones are organized, will migrate northward. This northward shift will be a consequence of the Northern Hemisphere, with its large continental area, warming faster than the Southern Hemisphere, with its large oceanic area. A third possibility is that both of these scenarios will play out simultaneously. We review paleoclimate evidence suggesting that (i) the middle latitudes were wetter during the last glacial maximum, (ii) a northward shift of the thermal equator attended the abrupt Bølling-Allerød climatic transition ~14.6 thousand years ago, and (iii) a southward shift occurred during the more recent Little Ice Age. We also inspect trends in seasonal surface heating between the hemispheres over the past several decades. From these clues, we predict that there will be a seasonally dependent response in rainfall patterns to global warming. During boreal summer, in which the rate of recent warming has been relatively uniform between the hemispheres, wet areas will get wetter and dry regions will become drier. During boreal winter, rain belts and drylands will expand northward in response to differential heating between the hemispheres.
Abstract
Premise
Many plant communities across the world are undergoing changes due to climate change, human disturbance, and other threats. These community‐level changes are often tracked with the ...use of permanent vegetative plots, but this approach is not always feasible. As an alternative, we propose using photogrammetry, specifically photograph‐based digital surface models (DSMs) developed using structure‐from‐motion, to establish virtual permanent plots in plant communities where the use of permanent structures may not be possible.
Methods
In 2021 and 2022, we took iPhone photographs to record species presence in 1‐m
2
plots distributed across alpine communities in the northeastern United States. We then compared field estimates of percent coverage with coverage estimated using DSMs.
Results
Digital surface models can provide effective, minimally invasive, and permanent records of plant species presence and percent coverage, while also allowing managers to mark survey locations virtually for long‐term monitoring. We found that percent coverage estimated from DSMs did not differ from field estimates for most species and substrates.
Discussion
In order to continue surveying efforts in areas where permanent structures or other surveying methods are not feasible, photogrammetry and structure‐from‐motion methods can provide a low‐cost approach that allows agencies to accurately survey and record sensitive plant communities through time.
The termination of the last ice age featured a major reconfiguration of Earthʼs climate and cryosphere, yet the underlying causes of these massive changes continue to be debated. Documenting the ...spatial and temporal variations of atmospheric temperature during deglaciation can help discriminate among potential drivers. Here, we present a 10Be surface-exposure chronology and glaciological reconstruction of ice recession following the Last Glacial Maximum (LGM) in the Rakaia valley, Southern Alps of New Zealand. Innermost LGM moraines at Big Ben have an age of 17,840 ± 240 yrs, whereas ice-marginal moraines or ice-molded bedrock surfaces at distances up-valley from Big Ben of 12.5 km (Lake Coleridge), ∼25 km (Castle Hill), ∼28 km (Double Hill), ∼43 km (Prospect Hill), and ∼58 km (Reischek knob) have ages of 17,020 ± 70 yrs, 17,100 ± 110 yrs, 16,960 ± 370 yrs, 16,250 ± 340 yrs, and 15,660 ± 160 yrs, respectively. These results indicate extensive recession of the Rakaia glacier, which we attribute primarily to the effects of climatic warming. In conjunction with geomorphological maps and a glaciological reconstruction for the Rakaia valley, we use our chronology to infer timing and magnitude of past atmospheric temperature changes. Compared to an overall temperature rise of ∼4.65 °C between the end of the LGM and the start of the Holocene, the glacier recession between ∼17,840 and ∼15,660 yrs ago is attributable to a net temperature increase of ∼4.0 °C (from −6.25 to −2.25 °C), accounting for ∼86% of the overall warming. Approximately 3.75 °C (∼70%) of the warming occurred between ∼17,840 and ∼16,250 yrs ago, with a further 0.75 °C (∼16%) increase between ∼16,250 and ∼15,660 yrs ago. A sustained southward shift of the Subtropical Front (STF) south of Australia between ∼17,800 and ∼16,000 yrs ago coincides with the warming over the Rakaia valley, and suggests a close link between Southern Ocean frontal boundary positions and southern mid-latitude climate. Most of the deglacial warming in the Southern Alps occurred during the early part of Heinrich Stadial 1 (HS1) of the North Atlantic region. Because the STF is associated with the position of the westerly wind belt, our findings support the concept that a southward shift of Earthʼs wind belts accompanied the early part of HS1 cooling in the North Atlantic, leading to warming and deglaciation in southern middle latitudes.
•Record of last deglaciation from the Rakaia valley, Southern Alps, New Zealand.•Extensive glacier recession between 17,840 and 15,660 yrs ago.•Atmospheric warming of 4 °C drove ice retreat.•Southern mid-latitude warming and glacier recession coeval with Heinrich Stadial 1.
The Younger Dryas Stadial (YDS; ∼12,900–11,600 y ago) in the Northern Hemisphere is classically defined by abrupt cooling and renewed glaciation during the last glacial–interglacial transition. ...Although this event involved a global reorganization of atmospheric and oceanic circulation Denton GH, Alley RB, Comer GC, Broecker WS (2005) Quat Sci Rev 24:1159–1182, the magnitude, seasonality, and geographical footprint of YDS cooling remain unresolved and pose a challenge to our understanding of abrupt climate change. Here, we present a deglacial chronology from Scotland, immediately downwind of the North Atlantic Ocean, indicating that the Scottish ice cap disintegrated during the first half of the YDS. We suggest that stratification of the North Atlantic Ocean resulted in amplified seasonality that, paradoxically, stimulated a severe wintertime climate while promoting warming summers through solar heating of the mixed layer. This latter process drove deglaciation of downwind landmasses to completion well before the end of the YDS.
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