Changes In Greenhouse Gas Concentrations Research Articles

Accelerating Large‐Eddy Simulations of Clouds With Tensor Processing Units

AbstractClouds, especially low clouds, are crucial for regulating Earth's energy balance and mediating the response of the climate system to changes in greenhouse gas concentrations. Despite their importance for climate, they remain relatively poorly understood and are inaccurately represented in climate models. A principal reason is that the high computational expense of simulating them with large‐eddy simulations (LES) has inhibited broad and systematic numerical experimentation and the generation of large data sets for training parametrization schemes for climate models. Here we demonstrate LES of low clouds on tensor processing units (TPUs), application‐specific integrated circuits that were originally developed for machine learning applications. We show that TPUs in conjunction with tailored software implementations can be used to simulate computationally challenging stratocumulus clouds in conditions observed during the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS) field study. The TPU‐based LES code successfully reproduces clouds during DYCOMS and opens up the large computational resources available on TPUs to cloud simulations. The code enables unprecedented weak and strong scaling of LES, making it possible, for example, to simulate stratocumulus with 10× speedup over real‐time evolution in domains with a 34.7 km × 53.8 km horizontal cross section. The results open up new avenues for computational experiments and for substantially enlarging the sample of LES available to train parameterizations of low clouds.

Future of birds nesting on river islands in the conditions of hydrological variability caused by climate change

The mew gull (Larus canus), little tern (Sternula albifrons) and black-headed gull (Chroicocephalus ridibundus) are threatened in Poland by the loss of breeding habitats due to changes in the hydrological regime of rivers and the frequency and length of inundation. Analysis of daily flows generated from the SWAT model allowed us to obtain the values of hydrological characteristics expressed as Indicators of Hydrological Alteration (IHA) and find the relationship with collected data on nesting success on islands and sandbanks in the Middle Vistula from 2004 until 2018. For each bird species, a set of adjusted IHA was calculated for future scenarios (2021-2050 and 2071-2100). The projections were prepared on the basis of EURO-CORDEX and contain two scenarios of changes in greenhouse gas concentrations: RCP4.5 and RCP8.5. Catastrophic breeding seasons quantification was carried out to assess the number of years that will have unsuitable hydrological breeding conditions in the projected climate change. The mew gull noted the lowest nesting success (during 2004-2018) but it seems that hydrology is not the principal factor causing it. This species will experience an increase in high flows due to climate change in the far future scenarios. The black-headed gull is projected not to be affected by an increase in the percentage of catastrophic breeding seasons due to climate change. The little tern seems to be the most affected by projected climate change due to an increase in high flows and, in consequence, an increasing percentage of catastrophic breeding seasons. The results confirmed the importance of hydrologic change for avian nesting success.

Temporal and Spatial Variation Characteristics of Methane, Carbon Dioxide, and Nitrous Oxide Concentrations and the Influencing Factors in Small Aquaculture Pond

As an important source of greenhouse gases, the changes in greenhouse gas concentrations of aquaculture ponds are not only the basis for accurate quantification of greenhouse gases emissions but are also important for identifying their influencing factors. The spatial and temporal variation characteristics of CH4, CO2, and N2O concentrations and the influencing factors in a typical small aquaculture pond in the Yangtze River Delta were analyzed based on the headspace equilibrium-gas chromatograph method. Except in spring, the concentrations of CH4, and N2O appeared high at noon or afternoon and were influenced by water temperature. Impacted by water temperature and aquatic plant photosynthesis, the concentrations of CO2 were high in the morning when photosynthesis was weak. The concentrations of CH4 and CO2 were the highest in autumn and the lowest in winter. The mean concentrations of CH4 in autumn and winter were 176.34 nmol·L-1 and 32.75 nmol·L-1, respectively, which were mainly affected by air temperature, water temperature, and dissolved oxygen. The average CO2 concentrations in autumn and winter were 134.37 μmol·L-1 and 23.10 μmol·L-1, respectively, and were mainly affected by aquatic vegetation photosynthesis and pH. N2O concentration was the highest in summer and the lowest in winter, with mean values of 97.05 nmol·L-1 and 19.41 nmol·L-1, respectively, which were mainly affected by air temperature and water temperature. In terms of the vertical spatial variations of the three greenhouse gases, the concentration of CH4decreased with water depth in summer, and the concentration differences between the surface layer and the bottom and middle layers were 71.28 nmol·L-1 and 42.80 nmol·L-1, respectively. The concentration of CH4 increased with water depth in autumn, and the concentration difference between the bottom layer and surface layer was 163.94 nmol·L-1. The CO2 concentration increased with water depth in summer and autumn. The concentration differences between the bottom and surface concentrations were 18.69 μmol·L-1 and 29.90 μmol·L-1, respectively. N2O concentration showed no obvious change in the vertical direction. For the horizontal variations, the concentrations of CH4, CO2, and N2O in the feeding area in summer and in chicken manure in spring were approximately 1.34-1.98 times and 1.95-2.42 times those in other areas, respectively, and the concentrations of N2O and CO2 in spring and summer were approximately 1.13-1.26 times and 1.39-1.74 times those in other areas.

Characteristics and source analysis of greenhouse gas concentration changes at Akedala Station in Central Asia

Mole fractions of atmospheric carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and sulfur hexafluoride (SF6) have been continuously measured since September 2009 at the Akedala Station (47°06′N, 87°58′E, 563.3 masl) in China. The station is located in the Central Asia and northwest of China, and it is the only station in that region with background conditions for long-term greenhouse gas observations. Characteristics of the mole fractions, growth rates, and influence of long-distance transport were studied considering data from September 2009 to December 2019. The greenhouse gases concentrations at Akedala Station show a trend of year-on-year growth, with CO2 concentrations ranging from 389.80 × 10−6 to 408.79 × 10−6 (molar fraction of substances, same below), CH4 concentrations ranging from 1890.07 × 10−9 to 1976.32 × 10−9, N2O concentrations ranging from 321.26 × 10−9 to 332.03 × 10−9, and SF6 concentrations ranging from 7.04 × 10−12 to 10.07 × 10−12, the growth rate of which is similar to the decadal average growth rate in the Northern Hemisphere. There exist obvious seasonal variations, with CO2 concentrations showing high in winter and low in summer and CH4 showing a distinct “W”-shaped trend while N2O and SF6 showing little difference between the four seasons. A relatively strong correlation and homology exist among the four greenhouse gases except in summer, and the analysis based on backward trajectories model shows that the Akedala Station is influenced by the airflow from northwest or southwest throughout the year. The Akedala Station is an important atmospheric background station in Central Asia, and its greenhouse gas concentration levels and variation characteristics are significantly different from those of the background stations in the monsoon region. Its degree changes are closely related to local source emissions, monsoon transport, and atmospheric photochemical processes.

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH4) fluxes at tree stems – a general phenomenon for plant systems?

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH₄) fluxes at tree stems – a general phenomenon for plant systems?

Greenhouse Gas Concentration and Volcanic Eruptions Controlled the Variability of Terrestrial Carbon Uptake Over the Last Millennium.

The terrestrial net biome production (NBP) is considered as one of the major drivers of interannual variation in atmospheric CO2 levels. However, the determinants of variability in NBP under the background climate (i.e., preindustrial conditions) remain poorly understood, especially on decadal‐to‐centennial timescales. We analyzed 1,000‐year simulations spanning 850‐1,849 from the Community Earth System Model (CESM) and found that the variability in NBP and heterotrophic respiration (RH) were largely driven by fluctuations in the net primary production (NPP) and carbon turnover rates in response to climate variability. On interannual to multidecadal timescales, variability in NBP was dominated by variation in NPP, while variability in RH was driven by variation in turnover rates. However, on centennial timescales (100‐1,000 years), the RH variability became more tightly coupled to that of NPP. The NBP variability on centennial timescales was low, due to the near cancellation of NPP and NPP‐driven RH changes arising from climate internal variability and external forcings: preindustrial greenhouse gases, volcanic eruptions, land use changes, orbital change, and solar activity. Factorial experiments showed that globally on centennial timescales, the forcing of changes in greenhouse gas concentrations were the largest contributor (51%) to variations in both NPP and RH, followed by volcanic eruptions impacting NPP (25%) and RH (31%). Our analysis of the carbon‐cycle suggests that geoengineering solutions by injection of stratospheric aerosols might be ineffective on longer timescales.

Oxygen minimum zone-type biogeochemical cycling in the Cenomanian-Turonian Proto-North Atlantic across Oceanic Anoxic Event 2

Oceanic Anoxic Events (OAEs) in Earth's history are regarded as analogues for current and future ocean deoxygenation, potentially providing information on its pacing and internal dynamics. In order to predict the Earth system's response to changes in greenhouse gas concentrations and radiative forcing, a sound understanding of how biogeochemical cycling differs in modern and ancient marine environments is required. Here, we report proxy records for iron (Fe), sulfur and nitrogen cycling in the Tarfaya upwelling system in the Cretaceous Proto-North Atlantic before, during and after OAE2 (∼93 Ma). We apply a novel quantitative approach to sedimentary Fe speciation, which takes into account the influence of terrigenous weathering and sedimentation as well as authigenic Fe (non-terrigenous, precipitated onsite) rain rates on Fe-based paleo-redox proxies. Generally elevated ratios of reactive Fe (i.e., bound to oxide, carbonate and sulfide minerals) to total Fe (FeHR/FeT) throughout the 5 million year record are attributed to transport-limited chemical weathering under greenhouse climate conditions. Trace metal and nitrogen isotope systematics indicate a step-wise transition from oxic to nitrogenous to euxinic conditions over several million years prior to OAE2. Taking into consideration the low terrigenous sedimentation rates in the Tarfaya Basin, we demonstrate that highly elevated FeHR/FeT from the mid-Cenomanian through OAE2 were generated with a relatively small flux of additional authigenic Fe. Evaluation of mass accumulation rates of reactive Fe in conjunction with the extent of pyritization of reactive Fe reveals that authigenic Fe and sulfide precipitation rates in the Tarfaya Basin were similar to those in modern upwelling systems. Because of a smaller seawater nitrate inventory, however, chemolithoautotrophic sulfide oxidation with nitrate was less efficient in preventing hydrogen sulfide release into the water column. As terrigenous weathering and sediment flux determine how much authigenic Fe is required to generate an anoxic euxinic or ferruginous proxy signature, we emphasize that both have to be taken into account when interpreting Fe-based paleo-redox proxies.

Role of resolution in regional climate change projections over China

This paper investigates the sensitivity of projected future climate changes over China to the horizontal resolution of a regional climate model RegCM4.4 (RegCM), using RCP8.5 as an example. Model validation shows that RegCM performs better in reproducing the spatial distribution and magnitude of present-day temperature, precipitation and climate extremes than the driving global climate model HadGEM2-ES (HadGEM, at 1.875° × 1.25° degree resolution), but little difference is found between the simulations at 50 and 25 km resolutions. Comparison with observational data at different resolutions confirmed the added value of the RCM and finer model resolutions in better capturing the probability distribution of precipitation. However, HadGEM and RegCM at both resolutions project a similar pattern of significant future warming during both winter and summer, and a similar pattern of winter precipitation changes including dominant increase in most areas of northern China and little change or decrease in the southern part. Projected precipitation changes in summer diverge among the three models, especially over eastern China, with a general increase in HadGEM, little change in RegCM at 50 km, and a mix of increase and decrease in RegCM at 25 km resolution. Changes of temperature-related extremes (annual total number of daily maximum temperature > 25 °C, the maximum value of daily maximum temperature, the minimum value of daily minimum temperature in the three simulations especially in the two RegCM simulations are very similar to each other; so are the precipitation-related extremes (maximum consecutive dry days, maximum consecutive 5-day precipitation and extremely wet days’ total amount). Overall, results from this study indicate a very low sensitivity of projected changes in this region to model resolution. While fine resolution is critical for capturing the spatial variability of the control climate, it may not be as important for capturing the climate response to homogeneous forcing (in this case greenhouse gas concentration changes).

Isolating the roles of different forcing agents in global stratospheric temperature changes using model integrations with incrementally added single forcings.

Satellite instruments show a cooling of global stratospheric temperatures over the whole data record (1979-2014). This cooling is not linear, and includes two descending steps in the early 1980s and mid-1990s. The 1979-1995 period is characterized by increasing concentrations of ozone depleting substances (ODS) and by the two major volcanic eruptions of El Chichón (1982) and Mount Pinatubo (1991). The 1995-present period is characterized by decreasing ODS concentrations and by the absence of major volcanic eruptions. Greenhouse gas (GHG) concentrations increase over the whole time period. In order to isolate the roles of different forcing agents in the global stratospheric temperature changes, we performed a set of AMIP-style simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). We find that in our model simulations the cooling of the stratosphere from 1979 to present is mostly driven by changes in GHG concentrations in the middle and upper stratosphere and by GHG and ODS changes in the lower stratosphere. While the cooling trend caused by increasing GHGs is roughly constant over the satellite era, changing ODS concentrations cause a significant stratospheric cooling only up to the mid-1990s, when they start to decrease because of the implementation of the Montreal Protocol. Sporadic volcanic events and the solar cycle have a distinct signature in the time series of stratospheric temperature anomalies but do not play a statistically significant role in the long-term trends from 1979 to 2014. Several factors combine to produce the step-like behavior in the stratospheric temperatures: in the lower stratosphere, the flattening starting in the mid 1990's is due to the decrease in ozone depleting substances; Mount Pinatubo and the solar cycle cause the abrupt steps through the aerosol-associated warming and the volcanically induced ozone depletion. In the middle and upper stratosphere, changes in solar irradiance are largely responsible for the step-like behavior of global temperatures anomalies, together with volcanically induced ozone depletion and water vapor increases in the post-Pinatubo years.

Accelerated greenhouse gases versus slow insolation forcing induced climate changes in southern South America since the Mid-Holocene

This paper is a pioneering analysis of past climates in southern South America combining multiproxy reconstructions and the state-of-the-art CMIP5/PMIP3 paleoclimatic models to investigate the time evolution of regional climatic conditions from the Mid-Holocene (MH) to the present. This analysis allows a comparison between the impact of the long term climate variations associated with insolation changes and the more recent effects of anthropogenic forcing on the region. The PMIP3 multimodel experiments suggest that changes in precipitation over almost all southern South America between MH and pre-industrial (PI) times due to insolation variations are significantly larger than those between PI and the present, which are due to changes in greenhouse gas concentrations. Anthropogenic forcing has been particularly intense over western Patagonia inducing reduction of precipitation in summer, autumn and winter as a consequence of progressively weaker westerly winds over the region, which have moved further poleward, between ca. 35–55°S and have become stronger south of about 50°S. Orbital variations between the MH to the PI period increased insolation over southern South America during summer and autumn inducing warmer conditions in the PI, accentuated by the effect of anthropogenic forcing during the last century. On the other hand, changes in orbital parameters from the MH to the PI period reduced insolation during winter and spring inducing colder conditions, which have been reversed by the anthropogenic forcing.

Disentangling greenhouse warming and aerosol cooling to reveal Earth’s climate sensitivity

Earth’s climate sensitivity has been debated. An econometric analysis of observations shows that aerosol cooling has masked about one-third of greenhouse gas warming and yields a transient climate sensitivity of 2 ± 0.8 °C. Earth’s climate sensitivity has long been subject to heated debate and has spurred renewed interest after the latest IPCC assessment report suggested a downward adjustment of its most likely range1. Recent observational studies have produced estimates of transient climate sensitivity, that is, the global mean surface temperature increase at the time of CO2 doubling, as low as 1.3 K (refs 2,3), well below the best estimate produced by global climate models (1.8 K). Here, we present an observation-based study of the time period 1964 to 2010, which does not rely on climate models. The method incorporates observations of greenhouse gas concentrations, temperature and radiation from approximately 1,300 surface sites into an energy balance framework. Statistical methods commonly applied to economic time series are then used to decompose observed temperature trends into components attributable to changes in greenhouse gas concentrations and surface radiation. We find that surface radiation trends, which have been largely explained by changes in atmospheric aerosol loading, caused a cooling that masked approximately one-third of the continental warming due to increasing greenhouse gas concentrations over the past half-century. In consequence, the method yields a higher transient climate sensitivity (2.0 ± 0.8 K) than other observational studies.

Near-linear response of mean monsoon strength to a broad range of radiative forcings

Theoretical models have been used to argue that seasonal mean monsoons will shift abruptly and discontinuously from wet to dry stable states as their radiative forcings pass a critical threshold, sometimes referred to as a "tipping point." Further support for a strongly nonlinear response of monsoons to radiative forcings is found in the seasonal onset of the South Asian summer monsoon, which is abrupt compared with the annual cycle of insolation. Here it is shown that the seasonal mean strength of monsoons instead exhibits a nearly linear dependence on a wide range of radiative forcings. First, a previous theory that predicted a discontinuous, threshold response is shown to omit a dominant stabilizing term in the equations of motion; a corrected theory predicts a continuous and nearly linear response of seasonal mean monsoon strength to forcings. A comprehensive global climate model is then used to show that the seasonal mean South Asian monsoon exhibits a near-linear dependence on a wide range of isolated greenhouse gas, aerosol, and surface albedo forcings. This model reproduces the observed abrupt seasonal onset of the South Asian monsoon but produces a near-linear response of the mean monsoon by changing the duration of the summer circulation and the latitude of that circulation's ascent branch. Thus, neither a physically correct theoretical model nor a comprehensive climate model support the idea that seasonal mean monsoons will undergo abrupt, nonlinear shifts in response to changes in greenhouse gas concentrations, aerosol emissions, or land surface albedo.

Influence of Sunspots on Global Mean Surface Temperature

The influence of variations in solar activity on earth's surface temperature is a key question in climate-change attribution. On the basis of natural cyclic sea surface temperatures and the annual mean daily sunspot number as a proxy for variability in solar influence on the energy retained by the planet, a predictive equation derived from well-understood physical principles proves capable of reproducing observed anomalies in annual global mean surface temperature since before 1900 with 90% accuracy (R2 = 0.9049). This accuracy results even where a zero-contribution from changes in greenhouse-gas concentrations is assumed. Here we show that the cumulative influence of the unusually elevated solar activity in the 64 years 1941–2005, as calculated using the time-integral of sunspot number anomalies (with a proxy factor) might have been a primary cause of the global warming observed through 2001. After 2001 the low solar activity is consistent with the observed flat average global temperature trend. The physically-based predictive equation, with the widely-projected decline in solar activity, moderated by the substantial effective thermal capacitance of the planet, especially the oceans, projects a downtrend steeper than 0.1K/decade.

National Mission for Climate Change Using Clean Development Mechanics

The success of our national efforts would be significantly enhanced provided the developed countries affirm their responsibility for accumulated green house gas emissions and fulfill their commitments under the United Nations framework Convention on Climate Change (UNFCCC) to transfer new and additional financial resources climate friendly technology to support both adaptation and mitigation in developing countries. Clean Development Mechanics (CDM) is a tool of weather patterns over periods ranging from decades to millions of years. CDM change in average weather conditions, or in the distribution of weather around the average conditions. It is caused by factors that include oceanic processes variations in solar radiation received by Earth, plate tectonics and volcanic eruption, and human-induced alterations of the natural world; these latter effects are currently using CDM “climate change” is often used to describe human-specific impacts. Scientists work to understand past and future climate by using observations and theoretical models. Temperature profiles, ice cores, floral and faunal records, glacial and per glacial processes, stable isotope and other sediment analyses, and sea level records serve to provide a climate record that spans the geologic past. More recent data are provided by the instrumental record. On other hand climate change risks are becoming biggest challenges of 21st century such as rising sea levels, that could force us to alter many of the ways economic and social activity are arranged. Climate-related changes may threaten human health, disrupt economic activity, damage natural ecosystems irreversibly, and even lead to mass migration, food shortage, and other global humanitarian crises. We have to outline the global climate strategy to curb it such as: conversion to lower carbon economies in both developed and developing countries, with market-based incentive schemes dominant in the former group, and low-carbon technology transfer schemes dominant in the latter. Engineering new and innovative form in the market in India using National Action Plan for Climate Change (NAPCC) is the rate at which energy is received from the sun and the rate at which it is lost to space determine the equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents, and other mechanisms to affect the climates of different regions. Factors that can shape climate are called climate forcing or “forcing mechanisms”. The Innovative project processes such as variations in solar radiation, variations in the Earth’s orbit, mountain- building and continental drift and changes in greenhouse gas concentrations. India faced with the challenge of sustaining its rapid economic growth while dealing with the global threat of climate change using CDM.

Oceanic Forcing of Antarctic Climate Change: A Study Using a Stretched-Grid Atmospheric General Circulation Model

Abstract A variable-resolution atmospheric general circulation model (AGCM) is used for climate change projections over the Antarctic. The present-day simulation uses prescribed observed sea surface conditions, while a set of five simulations for the end of the twenty-first century (2070–99) under the Special Report on Emissions Scenarios (SRES) A1B scenario uses sea surface condition anomalies from selected coupled ocean–atmosphere climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3). Analysis of the results shows that the prescribed sea surface condition anomalies have a very strong influence on the simulated climate change on the Antarctic continent, largely dominating the direct effect of the prescribed greenhouse gas concentration changes in the AGCM simulations. Complementary simulations with idealized forcings confirm these results. An analysis of circulation changes using self-organizing maps shows that the simulated climate change on regional scales is not principally caused by shifts of the frequencies of the dominant circulation patterns, except for precipitation changes in some coastal regions. The study illustrates that in some respects the use of bias-corrected sea surface boundary conditions in climate projections with a variable-resolution atmospheric general circulation model has some distinct advantages over the use of limited-area atmospheric circulation models directly forced by generally biased coupled climate model output.

Small influence of solar variability on climate over the past millennium

Climate variations over the past 1,000 years correspond to solar fluctuations, but the magnitude of the solar variability is unclear. An analysis of numerical simulations and climate reconstructions suggests that the amplitude of solar forcing was small over this interval, with the main climate forcing derived from volcanic eruptions and greenhouse gas concentrations. The climate of the past millennium was marked by substantial decadal and centennial scale variability in the Northern Hemisphere1. Low solar activity has been linked to cooling during the Little Ice Age (AD 1450–1850; ref. 1) and there may have been solar forcing of regional warmth during the Medieval Climate Anomaly2,3,4,5 (AD 950–1250; ref. 1). The amplitude of the associated changes is, however, poorly constrained5,6, with estimates of solar forcing spanning almost an order of magnitude7,8,9. Numerical simulations tentatively indicate that a small amplitude best agrees with available temperature reconstructions10,11,12,13. Here we compare the climatic fingerprints of high and low solar forcing derived from model simulations with an ensemble of surface air temperature reconstructions14 for the past millennium. Our methodology15 also accounts for internal climate variability and other external drivers such as volcanic eruptions, as well as uncertainties in the proxy reconstructions and model output. We find that neither a high magnitude of solar forcing nor a strong climate effect of that forcing agree with the temperature reconstructions. We instead conclude that solar forcing probably had a minor effect on Northern Hemisphere climate over the past 1,000 years, while, volcanic eruptions and changes in greenhouse gas concentrations seem to be the most important influence over this period.

Anthropogenic and natural causes of climate change

We test for causality between radiative forcing and temperature using multivariate time series models and Granger causality tests that are robust to the non-stationary (trending) nature of global climate data. We find that both natural and anthropogenic forcings cause temperature change and also that temperature causes greenhouse gas concentration changes. Although the effects of greenhouse gases and volcanic forcing are robust across model specifications, we cannot detect any effect of black carbon on temperature, the effect of changes in solar irradiance is weak, and the effect of anthropogenic sulfate aerosols may be only around half that usually attributed to them.

Increased sensitivity of the Plio-Pleistocene northwest Pacific to obliquity forcing

Insolation-driven changes in poleward heat transport and changes in greenhouse gas concentrations can explain aspects of the rise of obliquity-paced climate variability during the Pliocene–Pleistocene transition. Based on an alkenone-based sea surface temperature (SST) reconstruction of the Kuroshio Current Extension (KCE), we propose here that emergence of this region as the primary locus of ocean–atmosphere heat transfer in the Pacific Ocean promoted Northern Hemisphere Glaciation (NHG). Our record shows that with intensification of NHG at 2.7 Ma, the KCE cooled 2–4 °C during glacial intervals, likely in NH winter/spring. These high-amplitude 41-kyr SST cycles slightly lead δO18 variations, ruling out global ice-volume changes as the primary cause. The lead of SST over δO18 cycles matches the phasing between these two proxies observed in the tropics, supporting changes in CO2 concentrations as a unifying mechanism of ocean surface temperature change. However, the amplitude of the KCE SST cycle is twice that of the tropical records, pointing to an additional, regional process. We infer that cooling of the North Pacific sea surface by the East Asian winter monsoon and associated westerlies intensified during glacial intervals. This transfer of heat and moisture from the ocean to the atmosphere potentially furthered glacial formation by accelerating snow fall in North America. Therefore, these results might also support a role for tropical–extratropical heat balance in enhancing glacial growth via the obliquity pacing.

Modes of interannual variability of Southern Hemisphere atmospheric circulation in CMIP3 models: assessment and projections

An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere 500 hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project phase 3 (CMIP3) dataset. The analysis is done for both the intraseasonal and slow components of the geopotential height. When the CMIP3 models are assessed against reanalysis data, the spatial structure and variance of the leading modes in the intraseasonal component are generally well reproduced. There are systematic differences between the models in their reproduction of the leading modes in the slow component. An overall score using the leading modes in the slow component allows a categorisation of CMIP3 model performance. Using an ensemble from four models that suitably reproduce the twentieth century modes, modes of variability in the slow-internal and slow-external components are estimated. The leading mode of the slow-external component is shown to be related to observed changes in greenhouse gas concentrations. In this ensemble, there is little change in the leading modes in the intraseasonal component in the twenty-first century. Larger changes in variance, and subtle changes in regional-scale structure, are found for the leading modes in the slow-internal component. These are related to changes in the slowly varying dynamics of the Southern Annular Mode and the El Nino-Southern Oscillation. By far the biggest change is in the leading mode of the slow-external component. The spatial structure becomes uniform in the twenty-first century, and the variance increases with increasing greenhouse gas concentrations.

Comparison of 20th century and pre-industrial climate over South America in regional model simulations

Abstract. In this study, we assess how the anthropogenically induced increase in greenhouse gas concentrations affects the climate of central and southern South America. We utilise two regional climate simulations for present day (PD) and pre-industrial (PI) times. These simulations are compared to historical reconstructions in order to investigate the driving processes responsible for climatic changes between the different periods. The regional climate model is validated against observations for both re-analysis data and GCM-driven regional simulations for the second half of the 20th century. Model biases are also taken into account for the interpretation of the model results. The added value of the regional simulation over global-scale modelling relates to a better representation of hydrological processes that are particularly evident in the proximity of the Andes Mountains. Climatic differences between the simulated PD minus PI period agree qualitatively well with proxy-based temperature reconstructions, albeit the regional model overestimates the amplitude of the temperature increase. For precipitation the most important changes between the PD and PI simulation relate to a dipole pattern along the Andes Mountains with increased precipitation over the southern parts and reduced precipitation over the central parts. Here only a few regions show robust similarity with studies based on empirical evidence. However, from a dynamical point-of-view, atmospheric circulation changes related to an increase in high-latitude zonal wind speed simulated by the regional climate model are consistent with numerical modelling studies addressing changes in greenhouse gas concentrations. Our results indicate that besides the direct effect of greenhouse gas changes, large-scale changes in atmospheric circulation and sea surface temperatures also exert an influence on temperature and precipitation changes in southern South America. These combined changes in turn affect the relationship between climate and atmospheric circulation between PD and PI times and should be considered for the statistical reconstruction of climate indices calibrated within present-day climate data.