An abrupt drop in Northern Hemisphere sea surface temperature around 1970

Continental drift shifts tropical rainfall by altering radiation and ocean heat transport

Shifts in the position of the intertropical convergence zone (ITCZ) have great importance for weather, climate, and society. The ITCZ shifts have been extensively studied in current and future warmer climate; however, little is known for its migration in the past on geological time scales. Using an ensemble of climate simulations over the past 540 million years, we show that ITCZ migrations are controlled primarily by continental configuration through two competing pathways: hemispheric radiation asymmetry and cross-equatorial ocean heat transport. The hemispheric asymmetry of absorbed solar radiation is produced mainly by land-ocean albedo contrast, which can be predicted using only the landmass distribution. The cross-equatorial ocean heat transport is strongly associated with the hemispheric asymmetry of surface wind stress, which is, in turn, controlled by the hemispheric asymmetry of ocean surface area. These results allow the influence of continental evolution on global ocean-atmosphere circulations to be understood through simple mechanisms that depend primarily on the latitudinal distribution of land.

Spatial Variability in Years of Abrupt Seasonal Temperature Changes and Warming (Cooling) Hiatuses in China from 1951–2018 and the Variation Trends before and after These Years

Abrupt temperature changes and warming (cooling) hiatuses have an impact on the ecological environment. Currently, research findings for the spatial variability in the years of abrupt temperature changes and warming (cooling) hiatuses covering a variety of climate zones, as well as the variation trends before and after these years, are lacking. In the present study, based on the seasonal (monthly) average minimum temperatures, average temperatures, and average maximum temperature data from 622 Chinese meteorological stations during 1951–2018, the spatial variability in the years of abrupt seasonal changes and warming (cooling) hiatuses for these three temperature types in China, as well as the variation trends before and after these years, were analyzed using the Mann-Kendall test. The results are as follows. For most stations in China, the abrupt changes in the three temperature types during each season began to occur over a wide range in the late 1980s and early 1990s, and abrupt changes did not occur at a few stations concentrated south of 30° N. After an abrupt change occurred, the average minimum temperatures and average temperatures both showed significant upward trends, while the average maximum temperatures showed significant downward trends in some regions of southern China. After five to 15 years of temperature increases (decreases) following the abrupt changes, warming (cooling) hiatuses occurred in some areas of China, with the hiatus years occurring between 1989 and 2013. These hiatuses mainly occurred in 1998 and 2007, and in terms of proximity, the stations without warming (cooling) hiatuses were concentrated south of 40° N. After nine to 17 years of warming (cooling) hiatuses, the hiatuses ended at some stations between 2013 and 2017, after which the temperatures again increased rapidly. The periods of warming (cooling) hiatuses were longer in northern China than in southern China. Currently, there are some stations where the hiatuses have not ended, suggesting that the hiatus period is apparently longer than 17 years. The years of abrupt change, no abrupt change, hiatus, no hiatus, end of hiatus, and no end of hiatus, as well as their variation trends before and after these years, have shown strong spatial variability. The results of this study have enriched the research findings on climate change.

The Impact of Possible Decadal-Scale Cold Waves on Viticulture over Europe in a Context of Global Warming

A comprehensive analysis of all the possible impacts of future climate change is crucial for strategic plans of adaptation for viticulture. Assessments of future climate are generally based on the ensemble mean of state-of-the-art climate model projections, which prefigures a gradual warming over Europe for the 21st century. However, a few models project single or multiple O(10) year temperature drops over the North Atlantic due to a collapsing subpolar gyre (SPG) oceanic convection. The occurrence of these decadal-scale “cold waves” may have strong repercussions over the continent, yet their actual impact is ruled out in a multi-model ensemble mean analysis. Here, we investigate these potential implications for viticulture over Europe by coupling dynamical downscaled EUR-CORDEX temperature projections for the representative concentration pathways (RCP)4.5 scenario from seven different climate models—including CSIRO-Mk3-6-0 exhibiting a SPG convection collapse—with three different phenological models simulating the main developmental stages of the grapevine. The 21st century temperature increase projected by all the models leads to an anticipation of all the developmental stages of the grapevine, shifting the optimal region for a given grapevine variety northward, and making climatic conditions suitable for high-quality wine production in some European regions that are currently not. However, in the CSIRO-Mk3-6-0 model, this long-term warming trend is suddenly interrupted by decadal-scale cold waves, abruptly pushing the suitability pattern back to conditions that are very similar to the present. These findings are crucial for winemakers in the evaluation of proper strategies to face climate change, and, overall, provide additional information for long-term plans of adaptation, which, so far, are mainly oriented towards the possibility of continuous warming conditions.

Possible causes of data model discrepancy in the temperature history of the last Millennium

Model simulations and proxy-based reconstructions are the main tools for quantifying pre-instrumental climate variations. For some metrics such as Northern Hemisphere mean temperatures, there is remarkable agreement between models and reconstructions. For other diagnostics, such as the regional response to volcanic eruptions, or hemispheric temperature differences, substantial disagreements between data and models have been reported. Here, we assess the potential sources of these discrepancies by comparing 1000-year hemispheric temperature reconstructions based on real-world paleoclimate proxies with climate-model-based pseudoproxies. These pseudoproxy experiments (PPE) indicate that noise inherent in proxy records and the unequal spatial distribution of proxy data are the key factors in explaining the data-model differences. For example, lower inter-hemispheric correlations in reconstructions can be fully accounted for by these factors in the PPE. Noise and data sampling also partly explain the reduced amplitude of the response to external forcing in reconstructions compared to models. For other metrics, such as inter-hemispheric differences, some, although reduced, discrepancy remains. Our results suggest that improving proxy data quality and spatial coverage is the key factor to increase the quality of future climate reconstructions, while the total number of proxy records and reconstruction methodology play a smaller role.

Indian Ocean corals reveal crucial role of World War II bias for twentieth century warming estimates

The western Indian Ocean has been warming faster than any other tropical ocean during the 20^th century, and is the largest contributor to the global mean sea surface temperature (SST) rise. However, the temporal pattern of Indian Ocean warming is poorly constrained and depends on the historical SST product. As all SST products are derived from the International Comprehensive Ocean-Atmosphere dataset (ICOADS), it is challenging to evaluate which product is superior. Here, we present a new, independent SST reconstruction from a set of Porites coral geochemical records from the western Indian Ocean. Our coral reconstruction shows that the World War II bias in the historical sea surface temperature record is the main reason for the differences between the SST products, and affects western Indian Ocean and global mean temperature trends. The 20^th century Indian Ocean warming pattern portrayed by the corals is consistent with the SST product from the Hadley Centre (HadSST3), and suggests that the latter should be used in climate studies that include Indian Ocean SSTs. Our data shows that multi-core coral temperature reconstructions help to evaluate the SST products. Proxy records can provide estimates of 20^th century SST that are truly independent from the ICOADS data base.

Abrupt cooling over the North Atlantic in modern climate models

Observations over the 20th century evidence no long-term warming in the subpolar North Atlantic (SPG). This region even experienced a rapid cooling around 1970, raising a debate over its potential reoccurrence. Here we assess the risk of future abrupt SPG cooling in 40 climate models from the fifth Coupled Model Intercomparison Project (CMIP5). Contrary to the long-term SPG warming trend evidenced by most of the models, 17.5% of the models (7/40) project a rapid SPG cooling, consistent with a collapse of the local deep-ocean convection. Uncertainty in projections is associated with the models’ varying capability in simulating the present-day SPG stratification, whose realistic reproduction appears a necessary condition for the onset of a convection collapse. This event occurs in 45.5% of the 11 models best able to simulate the observed SPG stratification. Thus, due to systematic model biases, the CMIP5 ensemble as a whole underestimates the chance of future abrupt SPG cooling, entailing crucial implications for observation and adaptation policy.

Concerns on climate change include the risk of abrupt cooling in the North Atlantic. Here, the authors analyse CMIP5 projections and show that a convection collapse in the subpolar gyre can cool this region by up to 3°C in 10 years, which is as likely to occur by 2100 as a continuous warming.

Climate influence on Vibrio and associated human diseases during the past half-century in the coastal North Atlantic

Climate change is having a dramatic impact on marine animal and plant communities but little is known of its influence on marine prokaryotes, which represent the largest living biomass in the world oceans and play a fundamental role in maintaining life on our planet. In this study, for the first time to our knowledge, experimental evidence is provided on the link between multidecadal climatic variability in the temperate North Atlantic and the presence and spread of an important group of marine prokaryotes, the vibrios, which are responsible for several infections in both humans and animals. Using archived formalin-preserved plankton samples collected by the Continuous Plankton Recorder survey over the past half-century (1958-2011), we assessed retrospectively the relative abundance of vibrios, including human pathogens, in nine areas of the North Atlantic and North Sea and showed correlation with climate and plankton changes. Generalized additive models revealed that long-term increase in Vibrio abundance is promoted by increasing sea surface temperatures (up to ∼1.5 °C over the past 54 y) and is positively correlated with the Northern Hemisphere Temperature (NHT) and Atlantic Multidecadal Oscillation (AMO) climatic indices (P < 0.001). Such increases are associated with an unprecedented occurrence of environmentally acquired Vibrio infections in the human population of Northern Europe and the Atlantic coast of the United States in recent years.

Migrations and dynamics of the intertropical convergence zone

Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Niño events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.

Arctic climate tipping points

There is widespread concern that anthropogenic global warming will trigger Arctic climate tipping points. The Arctic has a long history of natural, abrupt climate changes, which together with current observations and model projections, can help us to identify which parts of the Arctic climate system might pass future tipping points. Here the climate tipping points are defined, noting that not all of them involve bifurcations leading to irreversible change. Past abrupt climate changes in the Arctic are briefly reviewed. Then, the current behaviour of a range of Arctic systems is summarised. Looking ahead, a range of potential tipping phenomena are described. This leads to a revised and expanded list of potential Arctic climate tipping elements, whose likelihood is assessed, in terms of how much warming will be required to tip them. Finally, the available responses are considered, especially the prospects for avoiding Arctic climate tipping points.