Query: “The jet stream forms a boundary between the cold north and the warmer south, but the lower temperature difference means the winds are now weaker.

Relevant passages:
Jet stream <=SEP=> Jet streams are fast flowing , narrow , meandering air currents found in the atmosphere of some planets , including Earth . On Earth , the main jet streams are located near the altitude of the tropopause and are westerly winds ( flowing west to east ) . Their paths typically have a meandering shape . Jet streams may start , stop , split into two or more parts , combine into one stream , or flow in various directions including opposite to the direction of the remainder of the jet . The strongest jet streams are the polar jets , at 9 - above sea level , and the higher altitude and somewhat weaker subtropical jets at 10 - . The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet . The northern hemisphere polar jet flows over the middle to northern latitudes of North America , Europe , and Asia and their intervening oceans , while the southern hemisphere polar jet mostly circles Antarctica all year round .   Jet streams are the product of two factors : the atmospheric heating by solar radiation that produces the large scale Polar , Ferrel , and Hadley circulation cells , and the action of the Coriolis force acting on those moving masses . The Coriolis force is caused by the planet 's rotation on its axis . On other planets , internal heat rather than solar heating drives their jet streams . The Polar jet stream forms near the interface of the Polar and Ferrel circulation cells ; while the subtropical jet forms near the boundary of the Ferrel and Hadley circulation cells .   Other jet streams also exist . During the Northern Hemisphere summer , easterly jets can form in tropical regions , typically where dry air encounters more humid air at high altitudes . Low-level jets also are typical of various regions such as the central United States .   Meteorologists use the location of some of the jet streams as an aid in weather forecasting . The main commercial relevance of the jet streams is in air travel , as flight time can be dramatically affected by either flying with the flow or against , which results in significant fuel and time cost savings for airlines . Often , the airlines work to fly ` with ' the jet stream for this reason . Dynamic North Atlantic Tracks are one example of how airlines and air traffic control work together to accommodate the jet steam and winds aloft that results in the maximum benefit for airlines and other users . Clear-air turbulence , a potential hazard to aircraft passenger safety , is often found in a jet stream 's vicinity , but it does not create a substantial alteration on flight times .

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 .