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Palaeoproxy evidence shows that the sea-surface-temperature gradient across the tropical Pacific Ocean strengthened during the Medieval Warm Period but weakens in model-projected future warming; this is because solar radiation induces greater precipitation for the same temperature change than greenhouse gases. The east–west gradient in tropical Pacific sea-surface temperatures is a key determinant of global rainfall patterns, but prior work has shown apparently contradictory responses to warming. The gradient strengthens in response to higher solar radiation (as in the Medieval Warm Period) but weakens as greenhouse gas levels increase (as in simulations of future climates). Reconciling these results has been problematic. Mark Cane and colleagues now show that the different forcings induce warming in different parts of the atmosphere, and that for the same warming, solar increases will induce overall higher precipitation than greenhouse gasses. As a result of global warming, precipitation is likely to increase in high latitudes and the tropics and to decrease in already dry subtropical regions1. The absolute magnitude and regional details of such changes, however, remain intensely debated2,3. As is well known from El Nino studies, sea-surface-temperature gradients across the tropical Pacific Ocean can strongly influence global rainfall4,5. Palaeoproxy evidence indicates that the difference between the warm west Pacific and the colder east Pacific increased in past periods when the Earth warmed as a result of increased solar radiation6,7,8,9. In contrast, in most model projections of future greenhouse warming this gradient weakens2,10,11. It has not been clear how to reconcile these two findings. Here we show in climate model simulations that the tropical Pacific sea-surface-temperature gradient increases when the warming is due to increased solar radiation and decreases when it is due to increased greenhouse-gas forcing. For the same global surface temperature increase the latter pattern produces less rainfall, notably over tropical land, which explains why in the model the late twentieth century is warmer than in the Medieval Warm Period (around ad 1000–1250) but precipitation is less. This difference is consistent with the global tropospheric energy budget12, which requires a balance between the latent heat released in precipitation and radiative cooling. The tropospheric cooling is less for increased greenhouse gases, which add radiative absorbers to the troposphere, than for increased solar heating, which is concentrated at the Earth’s surface. Thus warming due to increased greenhouse gases produces a climate signature different from that of warming due to solar radiation changes.

It has been proposed that changes in global climate have been responsible for episodes of widespread violence and even the collapse of civilizations. Yet previous studies have not shown that violence can be attributed to the global climate, only that random weather events might be correlated with conflict in some cases. Here we directly associate planetary-scale climate changes with global patterns of civil conflict by examining the dominant interannual mode of the modern climate, the El Niño/Southern Oscillation (ENSO). Historians have argued that ENSO may have driven global patterns of civil conflict in the distant past, a hypothesis that we extend to the modern era and test quantitatively. Using data from 1950 to 2004, we show that the probability of new civil conflicts arising throughout the tropics doubles during El Niño years relative to La Niña years. This result, which indicates that ENSO may have had a role in 21% of all civil conflicts since 1950, is the first demonstration that the stability of modern societies relates strongly to the global climate.

Forecasts of El Nino climate events are routinely provided and distributed, but the limits of El Nino predictability are still the subject of debate. Some recent studies suggest that the predictability is largely limited by the effects of high-frequency atmospheric 'noise', whereas others emphasize limitations arising from the growth of initial errors in model simulations. Here we present retrospective forecasts of the interannual climate fluctuations in the tropical Pacific Ocean for the period 1857 to 2003, using a coupled ocean-atmosphere model. The model successfully predicts all prominent El Nino events within this period at lead times of up to two years. Our analysis suggests that the evolution of El Nino is controlled to a larger degree by self-sustaining internal dynamics than by stochastic forcing. Model-based prediction of El Nino therefore depends more on the initial conditions than on unpredictable atmospheric noise. We conclude that throughout the past century, El Nino has been more predictable than previously envisaged.

Global climate change around 3‐4 Myr ago is thought to have influenced the evolution of hominids, via the aridification of Africa, and may have been the precursor to Pleistocene glaciation about 2.75 Myr ago. Most explanations of these climatic events involve changes in circulation of the North Atlantic Ocean due to the closing of the Isthmus of Panama. Here we suggest, instead, that closure of the Indonesian seaway 3‐4 Myr ago could be responsible for these climate changes, in particular the aridification of Africa. We use simple theory and results from an ocean circulation model to show that the northward displacement of New Guinea, about 5 Myr ago, may have switched the source of flow through Indonesia—from warm South Pacific to relatively cold North Pacific waters. This would have decreased sea surface temperatures in the Indian Ocean, leading to reduced rainfall over eastern Africa. We further suggest that the changes in the equatorial Pacific may have reduced atmospheric heat transport from the tropics to higher latitudes, stimulating global cooling and the eventual growth of ice sheets. Eastern Africa used to be more humid than it is now, and Canada used to be devoid of ice sheets; New Guinea lay south of its present position, and the island of Halmahera (part of Indonesia; see Fig. 1) lay mostly below sea level. The collision of Australia with the Banda arc occurred about 3 Myr ago, when Timor began to emerge within what had been the eastward continuation of the Java trench 1,2

Received wisdom about the main driver of the South Asian monsoon comes into question with a report that tests the idea that the Himalayas, not the Tibetan plateau, are the essential topographic ingredient. Heat emitted from the Tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the South Asian summer monsoon, but new work suggests that in fact it is the neighbouring mountains that are the major influence. William Boos and Zhiming Kuang use an atmospheric model to show that flattening the Tibetan plateau has little effect on the monsoon, so long as the Himalayas and surrounding mountain ranges remain. The plateau does boost rainfall locally along its southern edge, but it is the build-up of hot, moist air over India, insulated from colder, drier air by the Himalayas, that drives large-scale monsoon circulation.

Farmers in drought-prone regions of Andean South America have historically made observations of changes in the apparent brightness of stars in the Pleiades around the time of the southern winter solstice in order to forecast interannual variations in summer rainfall and in autumn harvests. They moderate the effect of reduced rainfall by adjusting the planting dates of potatoes, their most important crop. Here we use data on cloud cover and water vapour from satellite imagery, agronomic data from the Andean altiplano and an index of El Nino variability to analyse this forecasting method. We find that poor visibility of the Pleiades in June-caused by an increase in subvisual high cirrus clouds-is indicative of an El Nino year, which is usually linked to reduced rainfall during the growing season several months later. Our results suggest that this centuries-old method of seasonal rainfall forecasting may be based on a simple indicator of El Nino variability.

A deterministic numerical model of the coupled evolution of the tropical ocean and atmosphere was used to forecast all El Nino/Southern Oscillation (ENSO) events from 1970 to 1986. More particularly, the model, originally developed for studying large-scale ocean-atmosphere interactions in the tropics, successfully predicted the characteristics of the spatial and temporal structure of ENSO observed in the study interval. The model indicated that rainfall moving eastward over the Pacific slackens the surface winds that would otherwise cool the eastern Pacific by drawing up cooler subsurface waters. The oceanic thermocline increases, a poleward flow of westerly flowing warn waters deplets the equatorial warm water reservoir, and sea surface temperatures decline. These ENSO conditions are statistically tractable with the model several months in advance, provided upper ocean layer thermal data are available.