It has been difficult to access projections of global‐scale climate change with high temporal resolution spaning the late Pleistocene and Holocene. This has limited our ability to discern how climate ...fluctuations have affected species’ range dynamics and extinction processes, turn‐over in ecological communities and changes in genetic diversity. PaleoView is a new freeware tool, which provides a comprehensive but easy‐to‐use way to generate and view paleoclimate data at temporal and spatial resolutions suitable for detecting biotic responses to major climate shifts since the last glacial maximum. Regional to global scale simulations of temperature, precipitation, humidity and mean sea level pressure can be generated from PaleoView as gridded or time series data at time intervals as short as a decade for any period during the last 21 000 yr. They can be viewed using a built‐in geographical user interface or saved as data files. Modelled climate reconstructions are based on daily simulation output from the Community Climate System Model ver. 3 (CCSM3). This global coupled atmosphere–ocean–sea ice–land general circulation model accurately reproduces major climatic features associated with the most recent deglaciation event, and predicts present‐day patterns of climate conditions with verified hindcast skill. By providing a portal for readily accessing climate reconstructions at high temporal resolutions, PaleoView can help to better establish the consequences of past climate fluctuations on macro‐ecological patterns of biological and genetic diversity.
With CO2 concentrations similar to today (410 ppm), the Pliocene Epoch offers insights into climate changes under a moderately warmer world. Previous work suggested a low zonal sea surface ...temperature (SST) gradient in the tropical Pacific during the Pliocene, the so‐called “permanent El Niño.” Here, we recalculate SSTs using the alkenone proxy and find moderate reductions in both the zonal and meridional SST gradients during the mid‐Piacenzian warm period. These reductions are captured by coupled climate model simulations of the Pliocene, especially those that simulate weaker Walker circulation. We also produce a spatial reconstruction of mid‐Piacenzian warm period Pacific SSTs that closely resembles both Pliocene and future, low‐emissions simulations, a pattern that is, to a first order, diagnostic of weaker Walker circulation. Therefore, Pliocene warmth does not require drastic changes in the climate system—rather, it supports the expectation that the Walker circulation will weaken in the future under higher CO2.
Plain Language Summary
The Pliocene Epoch is the most recent time in Earth history when CO2 levels exceeded 400 ppm. The climate was warmer than preindustrial times, with smaller ice sheets. Previous studies suggested that the Pacific ocean was stuck in a “permanent El Niño” during the Pliocene. However, climate model simulations do not predict that this would happen at CO2 levels near 400 ppm—unusual changes in climate, such as large changes in cloud cover or hurricane frequency, would be needed to explain it. In this work we reanalyze Pliocene sea surface temperature data and do not find evidence of a permanent El Niño. Our results suggest that difference in temperatures across the tropical Pacific was smaller than it is today, but only by about 1 °C. Climate model simulations agree with our new analysis, suggesting that higher CO2, along with small changes in ice, vegetation, and mountains, is enough to explain Pliocene climate. We also show that the sea surface temperature patterns in the Pliocene Pacific Ocean look similar to those that climate models predict under a low‐emissions climate change scenario. The similarity suggests that the Pliocene can help us understand how the tropics respond to an ongoing increase in CO2.
Key Points
Pliocene SSTs calculated from the alkenone proxy do not support a “permanent El Niño”
Pliocene model simulations can reproduce proxy‐inferred SST patterns and gradients
The pattern of Pliocene warmth supports a weakening of Walker circulation under higher CO2
Under previous reconstructions of late Pliocene boundary conditions, climate models have failed to reproduce the warm sea surface temperatures reconstructed in the North Atlantic. Using a ...reconstruction of mid‐Piacenzian paleogeography that has the Bering Strait and Canadian Arctic Archipelago Straits closed, however, improves the simulation of the proxy‐indicated warm sea surface temperatures in the North Atlantic in the Community Climate System Model. We find that the closure of these small Arctic gateways strengthens the Atlantic Meridional Overturning Circulation, by inhibiting freshwater transport from the Pacific to the Arctic Ocean and from the Arctic Ocean to the Labrador Sea, leading to warmer sea surface temperatures in the North Atlantic. This indicates that the state of the Arctic gateways may influence the sensitivity of the North Atlantic climate in complex ways, and better understanding of the state of these Arctic gateways for past time periods is needed.
Key Points
Closure of Arctic gateways in a new reconstruction of mid‐Piacenzian paleogeography reduces simulated Arctic freshwater exports to the North Atlantic and enhances the AMOC
Simulated regional patterns of temperature show better correspondence with proxy‐indicated warm sea surface temperatures in the North Atlantic
The climatic response to the closure of Arctic gateways is not a linear combination to the closure of the individual straits
Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes. However, the causes of ...superposed century‐scale cold summer anomalies, of which the Little Ice Age (LIA) is the most extreme, remain debated, largely because the natural forcings are either weak or, in the case of volcanism, short lived. Here we present precisely dated records of ice‐cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430–1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium. A transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times, and that cold summers can be maintained by sea‐ice/ocean feedbacks long after volcanic aerosols are removed. Our results suggest that the onset of the LIA can be linked to an unusual 50‐year‐long episode with four large sulfur‐rich explosive eruptions, each with global sulfate loading >60 Tg. The persistence of cold summers is best explained by consequent sea‐ice/ocean feedbacks during a hemispheric summer insolation minimum; large changes in solar irradiance are not required.
Key Points
Little Ice Age began abruptly in two steps
Decadally paced explosive volcanism can explain the onset
A sea‐ice/ocean feedback can sustain the abrupt cooling
Holocene temperature conundrum Liu, Zhengyu; Zhu, Jiang; Rosenthal, Yair ...
Proceedings of the National Academy of Sciences - PNAS,
08/2014, Letnik:
111, Številka:
34
Journal Article
Recenzirano
Odprti dostop
A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene Marcott SA, et al., 2013, Science ...339(6124):1198–1201. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.
Sensitivity to Glacial Forcing in the CCSM4 Brady, Esther C.; Otto-Bliesner, Bette L.; Kay, Jennifer E. ...
Journal of climate,
03/2013, Letnik:
26, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Results are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the ...standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO₂ sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO₂) and to a quadrupling of the preindustrial concentration (4×CO₂), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of −6.2 W m−2. The radiative forcing includes the effects of the reduced LGM greenhouse gases, ice sheets, continental distribution with sea level lowered by approximately 120 m from the present, and orbital parameters, but not changes to atmospheric aerosols or vegetation biogeography. The LGM simulation has an equilibrium climate sensitivity (ECS) of 3.1(±0.3)°C, comparable to the CCSM4 4×CO₂ result. The LGMCO₂ simulation shows a greater ECS of 4.2°C. Other responses found at the LGM in CCSM4 include a global precipitation rate decrease at a rate of ∼2% °C−1, similar to climate change simulations in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4); a strengthening of the Atlantic meridional overturning circulation (AMOC) with a shoaling of North Atlantic Deep Water and a filling of the deep basin up to sill depth with Antarctic Bottom Water; and an enhanced seasonal cycle accompanied by reduced ENSO variability in the eastern Pacific Ocean’s SSTs.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A compilation of paleoceanographic data and a coupled atmosphere‐ocean climate model were used to examine global ocean surface temperatures of the Last Interglacial (LIG) period, and to produce the ...first quantitative estimate of the role that ocean thermal expansion likely played in driving sea level rise above present day during the LIG. Our analysis of the paleoclimatic data suggests a peak LIG global sea surface temperature (SST) warming of 0.7 ± 0.6°C compared to the late Holocene. Our LIG climate model simulation suggests a slight cooling of global average SST relative to preindustrial conditions (ΔSST = −0.4°C), with a reduction in atmospheric water vapor in the Southern Hemisphere driven by a northward shift of the Intertropical Convergence Zone, and substantially reduced seasonality in the Southern Hemisphere. Taken together, the model and paleoceanographic data imply a minimal contribution of ocean thermal expansion to LIG sea level rise above present day. Uncertainty remains, but it seems unlikely that thermosteric sea level rise exceeded 0.4 ± 0.3 m during the LIG. This constraint, along with estimates of the sea level contributions from the Greenland Ice Sheet, glaciers and ice caps, implies that 4.1 to 5.8 m of sea level rise during the Last Interglacial period was derived from the Antarctic Ice Sheet. These results reemphasize the concern that both the Antarctic and Greenland Ice Sheets may be more sensitive to temperature than widely thought.
Key Points
The thermal expansion component of Last Interglacial sea level rise was small
Antarctic Ice Sheets must have contributed 4.1 to 5.8 m of sea level rise
Polar ice sheets may be sensitive to small changes in global temperature
The response of the El Niño/Southern Oscillation (ENSO) to tropical volcanic eruptions has important worldwide implications, but remains poorly constrained. Paleoclimate records suggest an “El ...Niño-like” warming 1 year following major eruptions Adams JB, Mann ME, Ammann CM (2003) Nature 426:274–278 and “La Niña-like” cooling within the eruption year Li J, et al. (2013) Nat Clim Chang 3:822–826. However, climate models currently cannot capture all these responses. Many eruption characteristics are poorly constrained, which may contribute to uncertainties in model solutions—for example, the season of eruption occurrence is often unknown and assigned arbitrarily. Here we isolate the effect of eruption season using experiments with the Community Earth System Model (CESM), varying the starting month of two large tropical eruptions. The eruption-year atmospheric circulation response is strongly seasonally dependent, with effects on European winter warming, the Intertropical Convergence Zone, and the southeast Asian monsoon. This creates substantial variations in eruption-year hydroclimate patterns, which do sometimes exhibit La Niña-like features as in the proxy record. However, eruption-year equatorial Pacific cooling is not driven by La Niña dynamics, but strictly by transient radiative cooling. In contrast, equatorial warming the following year occurs for all starting months and operates dynamically like El Niño. Proxy reconstructions confirm these results: eruption-year cooling is insignificant, whereas warming in the following year is more robust. This implies that accounting for the event season may be necessary to describe the initial response to volcanic eruptions and that climate models may be more accurately simulating volcanic influences than previously thought.
Explosive volcanism is known to be a leading natural cause of climate change. The second half of the 13th century was likely the most volcanically perturbed half‐century of the last 2000 years, ...although none of the major 13th century eruptions have been clearly attributed to specific volcanoes. This period was in general a time of transition from the relatively warm Medieval period to the colder Little Ice Age, but available proxy records are insufficient on their own to clearly assess whether this transition is associated with volcanism. This context motivates our investigation of the climate system sensitivity to high‐ and low‐latitude volcanism using the fully coupled NCAR Community Climate System Model (CCSM3). We evaluate two sets of ensemble simulations, each containing four volcanic pulses, with the first set representing them as a sequence of tropical eruptions and the second representing eruptions occurring in the mid‐high latitudes of both the Northern and Southern hemispheres. The short‐term, direct radiative impacts of tropical and high‐latitude eruptions include significant cooling over the continents in summer and cooling over regions of increased sea‐ice concentration in Northern Hemisphere (NH) winter. A main dynamical impact of moderate tropical eruptions is a winter warming pattern across northern Eurasia. Furthermore, both ensembles show significant reductions in global precipitation, especially in the summer monsoon regions. The most important long‐term impact is the cooling of the high‐latitude NH produced by multiple tropical eruptions, suggesting that positive feedbacks associated with ice and snow cover could lead to long‐term climate cooling in the Arctic.
Monsoon responses to eruptions over the last millennium (LM) are examined in an ensemble of climate simulations as a function of eruption hemisphere. A composite analysis reveals a particularly ...strong sensitivity of monsoon rainfall in the year following Northern Hemisphere (NH) extratropical eruptions. Additional analysis focusing on the 18th century eruption of Mt. Laki and idealized simulations representing an analogue Southern Hemisphere eruption (SH‐Laki) reveal monsoon responses that are approximately symmetric across hemispheres, despite exhibiting asymmetries in other aspects of the climate response. We conclude that 1) latitudinally mirrored eruptions result in approximately symmetric monsoon responses, 2) disproportionate weakening (strengthening) of NH (SH) monsoons by NH eruptions over the LM resulted in part from their relatively high latitudes, and 3) uncertainty in eruption latitude fundamentally limits our ability to accurately simulate associated monsoon and tropical precipitation responses in nature.
Key Points
In idealized climate model experiments, hemispherically mirrored extratropical volcanic forcing is found to drive symmetric monsoon responses
The relatively high latitude of past northern hemisphere eruptions contributes to the relative severity of their monsoon responses
Uncertain aerosol latitudinal structures fundamentally limit our ability to simulate monsoon responses to volcanism in nature