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Global_JJ warming_NN1 is_VBZ simply_RR defined_VVN as_II the_AT steady_JJ increase_NN1 in_II the_AT Earth_NN1 's_GE surface_NN1 temperature_NN1 ._. 
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It_PPH1 is_VBZ considered_VVN to_TO be_VBI primarily_RR the_AT result_NN1 of_IO a_AT1 phenomenon_NN1 known_VVN as_II the_AT 'greenhouse_NN1 effect'_NN1 ._. 
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The_AT Earth_NN1 's_GE atmosphere_NN1 is_VBZ composed_VVN of_IO gases_NN2 ,_, as_CSA shown_VVN in_II Table_NN1 1_MC1 ._. 
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Some_DD of_IO these_DD2 gases_NN2 have_VH0 fixed_VVN proportions_NN2 ,_, such_II21 as_II22 nitrogen_NN1 and_CC oxygen_NN1 ,_, whilst_CS it_PPH1 is_VBZ the_AT greenhouse_NN1 gases_NN2 that_CST vary_VV0 in_II proportion_NN1 ._. 
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The_AT 'greenhouse_NN1 effect'_NN1 is_VBZ where_RRQ solar_JJ radiation_NN1 becomes_VVZ trapped_VVN in_II the_AT Earth_NN1 's_GE atmosphere_NN1 making_VVG the_AT atmosphere_NN1 warmer_JJR ._. 
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On_II the_AT whole_NN1 ,_, the_AT amount_NN1 of_IO solar_JJ radiation_NN1 absorbed_VVN by_II the_AT Earth_NN1 and_CC the_AT amount_NN1 reflected_VVD back_RP into_II space_NN1 balance_VV0 each_PPX221 other_PPX222 out_RP ,_, keeping_VVG the_AT Earth_NN1 's_GE surface_NN1 temperature_NN1 constant_NN1 ._. 
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Naturally_RR occurring_VVG greenhouse_NN1 gases_NN2 in_II the_AT atmosphere_NN1 such_II21 as_II22 carbon_NN1 dioxide_NN1 ,_, methane_NN1 and_CC water_NN1 vapour_NN1 absorb_VV0 long_JJ wave_NN1 terrestrial_JJ radiation_NN1 (_( reflected_VVN ,_, short_JJ wave_NN1 solar_JJ radiation_NN1 )_) ,_, thus_RR acting_VVG to_TO keep_VVI the_AT Earth_NN1 warm_JJ at_II an_AT1 average_JJ surface_NN1 temperature_NN1 of_IO 15oC_FO ._. 
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It_PPH1 is_VBZ only_RR when_CS concentrations_NN2 of_IO such_DA gases_NN2 increase_VV0 that_CST more_DAR radiation_NN1 can_VM be_VBI absorbed_VVN ._. 
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This_DD1 increased_JJ capability_NN1 of_IO the_AT Earth_NN1 's_GE atmosphere_NN1 to_TO absorb_VVI more_DAR radiation_NN1 has_VHZ led_VVN to_II a_AT1 term_NN1 known_VVN as_II the_AT ,_, 'enhanced_VVD greenhouse_NN1 effect'_NN1 ._. 
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This_DD1 means_VVZ that_CST the_AT Earth_NN1 retains_VVZ more_DAR heat_NN1 than_CSN is_VBZ being_VBG radiated_VVN back_RP into_II space_NN1 ,_, resulting_VVG in_II global_JJ warming_NN1 ._. 
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It_PPH1 is_VBZ known_VVN that_CST in_II the_AT Earth_NN1 's_GE history_NN1 ,_, the_AT average_JJ temperature_NN1 of_IO the_AT Earth_NN1 has_VHZ fluctuated_VVN constantly_RR ,_, resulting_VVG in_II periods_NN2 of_IO ice_NN1 ages_NN2 and_CC warmer_JJR interglacial_JJ periods_NN2 ._. 
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It_PPH1 appears_VVZ that_CST the_AT most_RGT recent_JJ ice_NN1 age_NN1 reached_VVD its_APPGE maximum_NN1 approximately_RR 20,000_MC years_NNT2 ago_RA ,_, and_CC that_CST the_AT Earth_NN1 is_VBZ now_RT in_II a_AT1 warmer_JJR interglacial_JJ period_NN1 ._. 
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However_RR in_II the_AT past_JJ century_NNT1 or_CC so_RR ,_, it_PPH1 has_VHZ been_VBN apparent_JJ that_CST the_AT Earth_NN1 is_VBZ experiencing_VVG an_AT1 unprecedented_JJ rate_NN1 of_IO warming_VVG (_( Graph_NN1 1_MC1 )_) ,_, and_CC evidence_NN1 collected_VVN suggests_VVZ that_CST human_JJ society_NN1 may_VM be_VBI to_TO blame_VVI ._. 
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Approximately_RR 200_MC years_NNT2 ago_RA ,_, Great_NP1 Britain_NP1 became_VVD the_AT first_MD country_NN1 to_TO industrialise_VVI ._. 
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The_AT primary_JJ cause_NN1 for_IF such_DA rapid_JJ growth_NN1 in_II industry_NN1 was_VBDZ the_AT utilisation_NN1 of_IO fossil_NN1 fuels_NN2 ._. 
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Fossil_NN1 fuels_NN2 include_VV0 coal_NN1 ,_, natural_JJ gas_NN1 and_CC oil_NN1 ,_, and_CC it_PPH1 was_VBDZ the_AT burning_JJ of_IO these_DD2 ,_, especially_RR coal_NN1 that_CST literally_RR powered_VVD the_AT Industrial_JJ Revolution_NN1 ._. 
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Since_II then_RT ,_, much_DA1 of_IO the_AT world_NN1 has_VHZ industrialised_JJ ,_, burning_VVG even_RR more_DAR fossil_NN1 fuels_NN2 ._. 
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Nowadays_RT ,_, fossil_NN1 fuels_NN2 supply_VV0 over_RG 80%_NNU of_IO the_AT World_NN1 's_GE energy_NN1 needs_NN2 ._. 
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However_RR ,_, the_AT burning_JJ of_IO such_DA fossil_NN1 fuels_NN2 is_VBZ having_VHG serious_JJ consequences_NN2 ._. 
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When_CS a_AT1 fossil_NN1 fuel_NN1 is_VBZ burned_VVN it_PPH1 releases_VVZ greenhouse_NN1 gases_NN2 ._. 
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The_AT most_RGT important_JJ of_IO these_DD2 gases_NN2 is_VBZ carbon_NN1 dioxide_NN1 ._. 
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Carbon_NN1 dioxide_NN1 is_VBZ a_AT1 greenhouse_NN1 gas_NN1 that_CST strongly_RR absorbs_VVZ infrared_JJ radiation_NN1 ._. 
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On_II average_NN1 it_PPH1 comprises_VVZ only_RR 0.0003%_FO or_CC 365ppmv_FO (_( parts_NN2 per_II million_NNO of_IO volume_NN1 )_) of_IO the_AT atmosphere_NN1 ,_, therefore_RR it_PPH1 can_VM be_VBI seen_VVN that_CST not_XX much_DA1 carbon_NN1 dioxide_NN1 released_VVN into_II the_AT atmosphere_NN1 is_VBZ required_VVN for_IF this_DD1 value_NN1 to_TO change_VVI significantly_RR ._. 
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Levels_NN2 of_IO carbon_NN1 dioxide_NN1 have_VH0 steadily_RR risen_VVN in_II the_AT atmosphere_NN1 in_II recent_JJ times_NNT2 (_( Graph_NN1 2_MC )_) ,_, levels_NN2 that_CST have_VH0 not_XX be_VBI seen_VVN on_II this_DD1 planet_NN1 for_IF millions_NNO2 of_IO years_NNT2 and_CC this_DD1 has_VHZ had_VHN the_AT consequence_NN1 of_IO warming_VVG the_AT atmosphere_NN1 ,_, in_RR21 particular_RR22 the_AT lower_JJR atmosphere_NN1 ._. 
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A_AT1 further_JJR consequence_NN1 of_IO this_DD1 is_VBZ that_CST as_II the_AT air_NN1 near_II the_AT surface_NN1 warms_VVZ ,_, more_DAR water_NN1 will_VM evaporate_VVI into_II the_AT air_NN1 from_II the_AT oceans_NN2 increasing_VVG the_AT water_NN1 vapour_NN1 content_NN1 of_IO the_AT air_NN1 ._. 
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The_AT excess_JJ water_NN1 vapour_NN1 ,_, which_DDQ itself_PPX1 is_VBZ the_AT most_RGT abundant_JJ greenhouse_NN1 gas_NN1 ,_, will_VM hence_RR increase_VVI the_AT greenhouse_NN1 effect_NN1 and_CC accelerate_VVI global_JJ warming_NN1 even_RR further_RRR ._. 
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This_DD1 is_VBZ known_VVN as_II the_AT water_NN1 vapour_NN1 greenhouse_NN1 effect_NN1 feedback_NN1 ._. 
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Methane_NN1 is_VBZ another_DD1 greenhouse_NN1 gas_NN1 ._. 
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It_PPH1 currently_RR has_VHZ a_AT1 concentration_NN1 of_IO 1.745_MC ppmv3_FO ,_, considerably_RR less_DAR than_CSN carbon_NN1 dioxide_NN1 ,_, but_CCB is_VBZ rising_VVG at_II a_AT1 rate_NN1 of_IO 0.6%_FO per_II year_NNT1 ._. 
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For_IF the_AT last_MD two_MC thousand_NNO years_NNT2 up_RP till_II 1800_MC however_RR ,_, this_DD1 value_NN1 was_VBDZ only_RR 0.7_MC ppmv4_FO ._. 
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<s>
It_PPH1 is_VBZ a_AT1 damaging_JJ greenhouse_NN1 gas_NN1 that_CST is_VBZ increasingly_RR becoming_VVG more_DAR of_IO a_AT1 concern_NN1 because_CS the_AT enhanced_JJ greenhouse_NN1 effect_NN1 caused_VVN by_II methane_NN1 is_VBZ about_II 7-8_MCMC times_NNT2 that_DD1 of_IO carbon_NN1 dioxide4_FO ._. 
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<s>
Methane_NN1 can_VM form_VVI naturally_RR ,_, such_II21 as_II22 in_II marshlands_NN2 ,_, but_CCB the_AT man-made_JJ sources_NN2 include_VV0 oil_NN1 wells_NN2 ,_, coal_NN1 mining_NN1 and_CC agricultural_JJ activities_NN2 ._. 
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<s>
Agricultural_JJ activities_NN2 are_VBR the_AT main_JJ source_NN1 of_IO methane_NN1 and_CC are_VBR from_II sources_NN2 such_II21 as_II22 rice_NN1 paddy_NN1 fields_NN2 ,_, enteric_JJ fermentation_NN1 from_II cattle_NN2 and_CC other_JJ livestock_NN ._. 
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<s>
Rice_NN1 forms_VVZ the_AT staple_NN1 diet_NN1 for_IF almost_RR 50%_NNU of_IO the_AT world_NN1 's_GE population_NN1 and_CC because_CS the_AT world_NN1 's_GE population_NN1 is_VBZ increasing_VVG exponentially_RR the_AT production_NN1 of_IO rice_NN1 in_II paddy_NN1 fields_NN2 is_VBZ increasing_VVG rapidly_RR too_RR ._. 
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This_DD1 can_VM therefore_RR only_RR mean_VVI that_CST concentrations_NN2 of_IO methane_NN1 will_VM continue_VVI to_TO increase_VVI ,_, exacerbating_VVG the_AT greenhouse_NN1 effect_NN1 ._. 
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One_MC1 other_JJ ,_, yet_RR minor_JJ greenhouse_NN1 gas_NN1 is_VBZ nitrous_JJ oxide_NN1 (_( laughing_VVG gas_NN1 )_) ._. 
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It_PPH1 forms_VVZ naturally_RR ,_, but_CCB can_VM also_RR be_VBI man-made_JJ as_RR21 well_RR22 ._. 
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Sources_NN2 include_VV0 fossil_NN1 fuel_NN1 combustion_NN1 ,_, nitrogenous_JJ fertilisers_NN2 and_CC animal_NN1 waste_NN1 ._. 
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On_II average_NN1 it_PPH1 has_VHZ a_AT1 concentration_NN1 of_IO about_RG 0.3_MC ppmv_NNU in_II the_AT atmosphere_NN1 ._. 
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When_CS compared_VVN to_II carbon_NN1 dioxide_NN1 ,_, this_DD1 amount_NN1 is_VBZ minimal_JJ ,_, but_CCB nitrous_JJ oxide_NN1 is_VBZ thought_VVN to_TO be_VBI responsible_JJ for_IF at_RR21 least_RR22 6%_NNU of_IO the_AT greenhouse_NN1 effect_NN1 so_CS far2_FO ,_, with_IW levels_NN2 of_IO this_DD1 gas_NN1 rising_VVG about_II 0.25%_FO per_II year1_FO ._. 
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<s>
It_PPH1 is_VBZ considered_VVN to_TO be_VBI of_IO great_JJ concern_NN1 because_CS the_AT lifetime_NNT1 of_IO nitrous_JJ oxide_NN1 is_VBZ about_RG 120_MC years1_FO ,_, which_DDQ means_VVZ that_CST this_DD1 gas_NN1 will_VM remain_VVI effective_JJ for_IF many_DA2 years_NNT2 to_TO come_VVI ._. 
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<s>
Another_DD1 greenhouse_NN1 gas_NN1 is_VBZ Chlorofluorocarbons_NP1 (_( CFCs_NN2 )_) ._. 
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<s>
They_PPHS2 are_VBR man_NN1 made_VVN chemicals_NN2 that_CST because_II21 of_II22 its_APPGE special_JJ properties_NN2 was_VBDZ widely_RR used_VVN in_II items_NN2 such_II21 as_II22 refrigerators_NN2 aerosol_NN1 spray_NN1 cans_NN2 ._. 
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<s>
Although_CS there_EX has_VHZ been_VBN a_AT1 complete_JJ ban_NN1 on_II using_VVG CFCs_NN2 ,_, the_AT effect_NN1 of_IO this_DD1 gas_NN1 when_CS it_PPH1 was_VBDZ in_II use_NN1 on_II the_AT atmosphere_NN1 was_VBDZ enormous_JJ ._. 
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The_AT average_JJ concentration_NN1 of_IO CFCs_NN2 is_VBZ currently_RR about_II 1ppbv_FO (_( parts_NN2 per_II billion_NNO by_II volume_NN1 )_) 1_MC1 ,_, this_DD1 is_VBZ incredibly_RR small_JJ ,_, even_RR compared_VVN to_II nitrous_JJ oxide_NN1 ,_, but_CCB a_AT1 single_JJ CFC_NP1 molecule_NN1 is_VBZ 10,000_MC times_NNT2 more_RRR effective_JJ at_II absorbing_JJ heat_NN1 than_CSN a_AT1 carbon_NN1 dioxide_NN1 molecule_NN1 and_CC is_VBZ why_RRQ CFCs_NN2 are_VBR a_AT1 significant_JJ enhancer_NN1 or_CC the_AT greenhouse_NN1 effect_NN1 ._. 
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CFCs_NN2 also_RR have_VH0 a_AT1 long_JJ lifetime_NNT1 and_CC destroy_VV0 ozone_NN1 ,_, a_AT1 gas_NN1 found_VVN in_II the_AT stratosphere_NN1 ._. 
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<s>
Although_CS this_DD1 gas_NN1 exists_VVZ in_RP very_RG small_JJ concentrations_NN2 ,_, 0.02-0.07_MCMC ppmv_NNU ,_, it_PPH1 protects_VVZ the_AT Earth_NN1 from_II harmful_JJ radiation_NN1 from_II the_AT Sun_NN1 ._. 
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Deforestation_NN1 also_RR plays_VVZ a_AT1 role_NN1 in_II enhancing_VVG the_AT greenhouse_NN1 effect_NN1 ._. 
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<s>
It_PPH1 's_VBZ responsible_JJ for_IF roughly_RR 25%_NNU of_IO all_DB carbon_NN1 dioxide_NN1 emissions_NN2 entering_VVG the_AT atmosphere_NN1 ,_, by_II the_AT burning_JJ and_CC clearing_NN1 of_IO approximately_RR 34_MC million_NNO acres_NNU2 of_IO trees_NN2 per_II year_NNT1 ._. 
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<s>
Trees_NN2 are_VBR capable_JJ of_IO storing_VVG vast_JJ amounts_NN2 of_IO carbon_NN1 dioxide_NN1 ,_, an_AT1 acre_NNU1 alone_JJ of_IO forest_NN1 can_VM store_VVI up_RG21 to_RG22 2.8_MC tonnes_NNU2 of_IO carbon_NN1 dioxide_NN1 ._. 
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When_RRQ forests_NN2 are_VBR cleared_VVN ,_, more_DAR carbon_NN1 dioxide_NN1 enters_VVZ the_AT atmosphere_NN1 as_CSA less_DAR is_VBZ absorbed_VVN ,_, thus_RR enhancing_VVG the_AT greenhouse_NN1 effect_NN1 ._. 
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Over_II the_AT past_JJ century_NNT1 ,_, there_EX is_VBZ evidence_NN1 to_TO suggest_VVI that_DD1 global_JJ warming_NN1 is_VBZ happening_VVG ._. 
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It_PPH1 has_VHZ been_VBN calculated_VVN using_VVG historic_JJ records_NN2 that_CST the_AT average_JJ surface_NN1 temperature_NN1 of_IO the_AT Earth_NN1 has_VHZ raised_VVN by_II 0.6_MC oC_NN1 in_II the_AT last_MD 100_MC years_NNT2 ._. 
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This_DD1 is_VBZ clearly_RR seen_VVN in_II graph_NN1 1_MC1 ._. 
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Although_CS this_DD1 increase_NN1 seems_VVZ negligible_JJ ,_, it_PPH1 is_VBZ a_AT1 significant_JJ rise_NN1 for_IF such_DA a_AT1 small_JJ time_NNT1 ._. 
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It_PPH1 has_VHZ also_RR been_VBN recorded_VVN that_CST 8_MC of_IO the_AT 10_MC warmest_JJT years_NNT2 since_II 1860_MC have_VH0 occurred_VVN during_II the_AT last_MD decade_NNT1 ._. 
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This_DD1 occurrence_NN1 is_VBZ remarkable_JJ and_CC indicates_VVZ that_CST the_AT Earth_NN1 is_VBZ warming_VVG at_II an_AT1 alarming_JJ rate_NN1 ._. 
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Warming_NN1 of_IO the_AT atmosphere_NN1 has_VHZ equally_RR resulted_VVN in_II the_AT warming_NN1 of_IO the_AT oceans_NN2 ._. 
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As_II the_AT oceans_NN2 warm_VV0 up_RP ,_, they_PPHS2 expand_VV0 ,_, causing_VVG sea_NN1 levels_NN2 to_TO gradually_RR rise_VVI ._. 
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In_II the_AT last_MD 100_MC years_NNT2 ,_, the_AT global_JJ sea_NN1 level_NN1 has_VHZ risen_VVN by_RP about_II 10_MC to_II 25_MC cm_NNU (_( Graph_NN1 3_MC )_) ._. 
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The_AT problem_NN1 is_VBZ intensified_VVN by_II melting_VVG ice_NN1 caps_NN2 at_II both_DB2 the_AT Arctic_JJ and_CC Antarctic_NP1 ._. 
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To_II date_NN1 ,_, Arctic_JJ sea_NN1 ice_NN1 has_VHZ shrunk_VVN by_RP about_II 250_MC million_NNO acres_NNU2 ._. 
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Ice_NN1 melting_VVG on_II such_DA a_AT1 large_JJ scale_NN1 adds_VVZ vast_JJ amounts_NN2 of_IO water_NN1 into_II the_AT oceans_NN2 ,_, accelerating_JJ sea_NN1 level_NN1 rise_NN1 that_CST threaten_VV0 many_DA2 low_JJ lying_NN1 coasts_NN2 across_II the_AT world_NN1 from_II serious_JJ flooding_NN1 ._. 
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<s>
More_RGR frequent_JJ ,_, extreme_JJ weather_NN1 events_NN2 are_VBR another_DD1 indicator_NN1 that_CST global_JJ warming_NN1 is_VBZ taking_VVG place_NN1 ._. 
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<s>
The_AT warming_NN1 of_IO the_AT atmosphere_NN1 and_CC the_AT oceans_NN2 has_VHZ in_II some_DD regions_NN2 of_IO the_AT world_NN1 has_VHZ enhanced_VVN the_AT hydrological_JJ cycle_NN1 ._. 
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<s>
This_DD1 has_VHZ produced_VVN more_RGR intense_JJ storms_NN2 and_CC hurricanes_NN2 ,_, extensive_JJ flooding_NN1 and_CC in_II some_DD parts_NN2 of_IO the_AT world_NN1 ,_, severe_JJ drought_NN1 ._. 
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It_PPH1 appears_VVZ that_CST extreme_JJ events_NN2 that_CST normally_RR occur_VV0 rarely_RR are_VBR becoming_VVG more_RGR common_JJ ._. 
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<s>
It_PPH1 has_VHZ also_RR been_VBN recorded_VVN that_CST the_AT majority_NN1 of_IO all_DB the_AT world_NN1 's_GE glaciers_NN2 are_VBR retreating_VVG ._. 
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The_AT rise_NN1 in_II lower_JJR atmospheric_JJ temperature_NN1 is_VBZ melting_VVG the_AT ice_NN1 of_IO the_AT glaciers_NN2 causing_VVG them_PPHO2 to_TO shrink_VVI ._. 
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The_AT Grinnell_NP1 Glacier_NP1 ,_, USA_NP1 ,_, has_VHZ already_RR retreated_VVN by_II 90%_NNU ._. 
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This_DD1 is_VBZ similar_JJ across_II the_AT world_NN1 ._. 
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Further_JJR evidence_NN1 of_IO global_JJ warming_NN1 can_VM also_RR be_VBI found_VVN in_II ice_NN1 cores_NN2 ._. 
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<s>
Analysis_NN1 of_IO ice_NN1 cores_NN2 taken_VVN from_II the_AT Dunde_NN1 Ice_NN1 Cap_NN1 in_II the_AT Qilian_NP1 Mountains_NNL2 ,_, northwest_ND1 China_NP1 indicate_VV0 that_CST the_AT years_NNT2 since_II 1938_MC have_VH0 been_VBN the_AT warmest_JJT in_II the_AT last_MD 12,000_MC years_NNT2 ._. 
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The_AT ocean_NN1 's_GE coral_NN1 ecosystems_NN2 are_VBR also_RR being_VBG harmed_VVN ._. 
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<s>
Coral_NN1 is_VBZ being_VBG bleached_VVN by_II rising_VVG ocean_NN1 temperatures_NN2 as_II the_AT water_NN1 is_VBZ simply_RR too_RG warm_JJ for_IF the_AT coral_NN1 to_TO survive_VVI ._. 
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Higher_JJR temperatures_NN2 are_VBR also_RR reducing_VVG the_AT calcium_NN1 content_NN1 of_IO the_AT water_NN1 ,_, limiting_JJ coral_NN1 growth_NN1 ._. 
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It_PPH1 has_VHZ been_VBN estimated_VVN that_CST 25%_NNU of_IO the_AT world_NN1 's_GE coral_NN1 reefs_NN2 have_VH0 already_RR been_VBN destroyed_VVN by_II global_JJ warming_NN1 and_CC pollution3_FO ._. 
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During_II the_AT past_JJ century_NNT1 ,_, concentrations_NN2 of_IO greenhouse_NN1 gases_NN2 such_II21 as_II22 carbon_NN1 dioxide_NN1 ,_, methane_NN1 and_CC water_NN1 vapour_NN1 have_VH0 risen_VVN sharply_RR in_II the_AT atmosphere_NN1 enhancing_VVG the_AT greenhouse_NN1 effect_NN1 ._. 
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As_II a_AT1 result_NN1 of_IO greater_JJR concentrations_NN2 of_IO these_DD2 gases_NN2 in_II the_AT atmosphere_NN1 ,_, more_DAR radiation_NN1 from_II the_AT sun_NN1 becomes_VVZ trapped_VVN thus_RR acting_VVG to_TO warm_VVI the_AT earth_NN1 ._. 
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<s>
The_AT last_MD 100_MC years_NNT2 has_VHZ experienced_VVN a_AT1 rate_NN1 of_IO warming_VVG never_RR witnessed_VVN before_II and_CC this_DD1 global_JJ warming_NN1 is_VBZ proving_VVG to_TO have_VHI consequences_NN2 for_IF both_DB2 the_AT environment_NN1 and_CC human_JJ society_NN1 ._. 
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<s>
Sea_NN1 levels_NN2 are_VBR rising_VVG ,_, the_AT world_NN1 's_GE weather_NN1 is_VBZ changing_VVG and_CC with_IW the_AT world_NN1 's_GE population_NN1 rapidly_RR increasing_JJ ,_, this_DD1 problem_NN1 is_VBZ set_VVN to_TO continue_VVI ._. 
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