Query: The extreme cycles of dry and wet weather appear to have been intensifying over the last three decades.

Relevant passages:
El Niño–Southern Oscillation <=SEP=> El Niño -- Southern Oscillation ( ENSO ) is an irregularly periodical variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean , affecting much of the tropics and subtropics . The warming phase is known as El Niño and the cooling phase as La Niña . Southern Oscillation is the accompanying atmospheric component , coupled with the sea temperature change : El Niño is accompanied with high , and La Niña with low air surface pressure in the tropical western Pacific . The two periods last several months each ( typically occur every few years ) and their effects vary in intensity .   The two phases relate to the Walker circulation , discovered by Gilbert Walker during the early twentieth century . The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean , and a low pressure system over Indonesia . When the Walker circulation weakens or reverses , an El Niño results , causing the ocean surface to be warmer than average , as upwelling of cold water occurs less or not at all . An especially strong Walker circulation causes a La Niña , resulting in cooler ocean temperatures due to increased upwelling .   Mechanisms that cause the oscillation remain under study . The extremes of this climate pattern 's oscillations cause extreme weather ( such as floods and droughts ) in many regions of the world . Developing countries dependent upon agriculture and fishing , particularly those bordering the Pacific Ocean , are the most affected .

Little Ice Age <=SEP=> The Little Ice Age ( LIA ) was a period of cooling that occurred after the Medieval Warm Period . Although it was not a true ice age , the term was introduced into scientific literature by François E. Matthes in 1939 . It has been conventionally defined as a period extending from the 16th to the 19th centuries , but some experts prefer an alternative timespan from about 1300 to about 1850 . Climatologists and historians working with local records no longer expect to agree on either the start or end dates of the period , which varied according to local conditions .   The NASA Earth Observatory notes three particularly cold intervals : one beginning about 1650 , another about 1770 , and the last in 1850 , all separated by intervals of slight warming . The Intergovernmental Panel on Climate Change Third Assessment Report considered the timing and areas affected by the Little Ice Age suggested largely-independent regional climate changes rather than a globally-synchronous increased glaciation . At most , there was modest cooling of the Northern Hemisphere during the period .   Several causes have been proposed : cyclical lows in solar radiation , heightened volcanic activity , changes in the ocean circulation , variations in Earth 's orbit and axial tilt ( orbital forcing ) , inherent variability in global climate , and decreases in the human population .

Global warming <=SEP=> Global warming , also referred to as climate change , is the observed century-scale rise in the average temperature of the Earth 's climate system and its related effects . Multiple lines of scientific evidence show that the climate system is warming . Many of the observed changes since the 1950s are unprecedented in the instrumental temperature record which extends back to the mid 19th century , and in paleoclimate proxy records over thousands of years .   In 2013 , the Intergovernmental Panel on Climate Change ( IPCC ) Fifth Assessment Report concluded that `` It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century . '' The largest human influence has been emission of greenhouse gases such as carbon dioxide , methane and nitrous oxide . Climate model projections summarized in the report indicated that during the 21st century the global surface temperature is likely to rise a further 0.3 to for their lowest emissions scenario and 2.6 to for the highest emissions scenario . These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing .   Future climate change and associated impacts will differ from region to region around the globe . Anticipated effects include warming global temperature , rising sea levels , changing precipitation , and expansion of deserts in the subtropics . Warming is expected to be greater over land than over the oceans and greatest in the Arctic , with the continuing retreat of glaciers , permafrost and sea ice . Other likely changes include more frequent extreme weather events including heat waves , droughts , heavy rainfall with floods and heavy snowfall ; ocean acidification ; and species extinctions due to shifting temperature regimes . Effects significant to humans include the threat to food security from decreasing crop yields and the abandonment of populated areas due to rising sea levels . Because the climate system has a large `` inertia '' and greenhouse gases will stay in the atmosphere for a long time , many of these effects will not only exist for decades or centuries , but will persist for tens of thousands of years .   Possible societal responses to global warming include mitigation by emissions reduction , adaptation to its effects , building systems resilient to its effects , and possible future climate engineering . Most countries are parties to the United Nations Framework Convention on Climate Change ( UNFCCC ) ,  whose ultimate objective is to prevent dangerous anthropogenic climate change . Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 C-change relative to pre-industrial levels , with efforts made to limit warming to 1.5 C-change .   Public reactions to global warming and concern about its effects are also increasing . A global 2015 Pew Research Center report showed a median of 54 % consider it `` a very serious problem '' . There are significant regional differences , with Americans and Chinese ( whose economies are responsible for the greatest annual CO2 emissions ) among the least concerned .

Effects of global warming <=SEP=> The effects of global warming are the environmental and social changes caused ( directly or indirectly ) by human emissions of greenhouse gases . There is a scientific consensus that climate change is occurring , and that human activities are the primary driver . Many impacts of climate change have already been observed , including glacier retreat , changes in the timing of seasonal events ( e.g. , earlier flowering of plants ) , and changes in agricultural productivity .   Future effects of climate change will vary depending on climate change policies and social development . The two main policies to address climate change are reducing human greenhouse gas emissions ( climate change mitigation ) and adapting to the impacts of climate change . Geoengineering is another policy option .   Near-term climate change policies could significantly affect long-term climate change impacts . Stringent mitigation policies might be able to limit global warming ( in 2100 ) to around 2 ° C or below , relative to pre-industrial levels . Without mitigation , increased energy demand and extensive use of fossil fuels might lead to global warming of around 4 ° C. Higher magnitudes of global warming would be more difficult to adapt to , and would increase the risk of negative impacts .

Sahara <=SEP=> The Sahara ( الصحراء الكبرى , , ` the Greatest Desert ' ) is the largest hot desert and the third largest desert in the world after Antarctica and the Arctic . Its area of 9200000 km2 is comparable to the area of the United States .   The desert comprises much of North Africa , excluding the fertile region on the Mediterranean Sea coast , the Atlas Mountains of the Maghreb , and the Nile Valley in Egypt and Sudan . It stretches from the Red Sea in the east and the Mediterranean in the north to the Atlantic Ocean in the west , where the landscape gradually changes from desert to coastal plains . To the south , it is bounded by the Sahel , a belt of semi-arid tropical savanna around the Niger River valley and the Sudan Region of Sub-Saharan Africa .   The Sahara can be divided into several regions including : the western Sahara , the central Ahaggar Mountains , the Tibesti Mountains , the Aïr Mountains , the Ténéré desert , and the Libyan Desert .   The name ` Sahara ' is derived from ( , pronounced -LSB- / / ˈSaHa : ra : -RSB- ) , the plural of the Arabic word for `` desert '' .