Global_JJ Climate_NN1 Change_NN1 impacts_NN2 in_II the_AT United_NP1 States_NP1 What_DDQ is_VBZ this_DD1 report_NN1 ?_? 
This_DD1 report_NN1 summarizes_VVZ the_AT science_NN1 of_IO climate_NN1 change_NN1 and_CC the_AT impacts_NN2 of_IO climate_NN1 change_NN1 on_II the_AT United_NP1 States_NP1 ,_, now_RT and_CC in_II the_AT future_NN1 ._. 
It_PPH1 is_VBZ largely_RR based_VVN on_II results_NN2 of_IO the_AT U.S._NP1 Global_JJ Change_NN1 Research_NN1 Program_NN1 (_( USGCRP_NP1 )_) ,_, and_CC integrates_VVZ those_DD2 results_NN2 with_IW related_JJ research_NN1 from_II around_II the_AT world_NN1 ._. 
This_DD1 report_NN1 discusses_VVZ climate-related_JJ impacts_NN2 for_IF various_JJ societal_JJ and_CC environmental_JJ sectors_NN2 and_CC regions_NN2 across_II the_AT nation_NN1 ._. 
It_PPH1 is_VBZ an_AT1 authoritative_JJ scientific_JJ report_NN1 written_VVN in_II plain_JJ language_NN1 ,_, with_IW the_AT goal_NN1 of_IO better_JJR informing_NN1 public_NN1 and_CC private_JJ decision_NN1 making_VVG at_II all_DB levels_NN2 ._. 
Who_PNQS called_VVD for_IF it_PPH1 ,_, who_PNQS wrote_VVD it_PPH1 ,_, and_CC who_PNQS approved_VVD it_PPH1 ?_? 
The_AT USGCRP_NN1 called_VVN for_IF this_DD1 report_NN1 ._. 
An_AT1 expert_NN1 team_NN1 of_IO scientists_NN2 operating_VVG under_II the_AT authority_NN1 of_IO the_AT Federal_JJ Advisory_JJ Committee_NN1 Act_NN1 ,_, assisted_VVN by_II communication_NN1 specialists_NN2 ,_, wrote_VVD the_AT document_NN1 ._. 
The_AT report_NN1 was_VBDZ extensively_RR reviewed_VVN and_CC revised_VVN based_VVN on_II comments_NN2 from_II experts_NN2 and_CC the_AT public_NN1 ._. 
The_AT report_NN1 was_VBDZ approved_VVN by_II its_APPGE lead_NN1 USGCRP_NN1 Agency_NN1 ,_, the_AT National_JJ Oceanic_JJ and_CC Atmospheric_JJ Administration_NN1 ,_, the_AT other_JJ USGCRP_NN1 agencies_NN2 ,_, and_CC the_AT Committee_NN1 on_II the_AT Environment_NN1 and_CC Natural_JJ Resources_NN2 on_II31 behalf_II32 of_II33 the_AT National_JJ Science_NN1 and_CC Technology_NN1 Council_NN1 ._. 
This_DD1 report_NN1 meets_VVZ all_DB Federal_JJ requirements_NN2 associated_VVN with_IW the_AT Information_NN1 Quality_NN1 Act_NN1 ,_, including_II those_DD2 pertaining_II21 to_II22 public_JJ comment_NN1 and_CC transparency_NN1 ._. 
What_DDQ are_VBR its_APPGE sources_NN2 ?_? 
The_AT report_NN1 draws_VVZ from_II a_AT1 large_JJ body_NN1 of_IO scientific_JJ information_NN1 ._. 
The_AT foundation_NN1 of_IO this_DD1 report_NN1 is_VBZ a_AT1 set_NN1 of_IO 21_MC Synthesis_NN1 and_CC Assessment_NN1 Products_NN2 (_( SAPs_VVZ )_) ,_, which_DDQ were_VBDR designed_VVN to_TO address_VVI key_JJ policy-relevant_JJ issues_NN2 in_II climate_NN1 science_NN1 (_( see_VV0 page_NN1 161_MC )_) ;_; several_DA2 of_IO these_DD2 were_VBDR also_RR summarized_VVN in_II the_AT Scientific_JJ Assessment_NN1 of_IO the_AT Effects_NN2 of_IO Climate_NN1 Change_NN1 on_II the_AT United_NP1 States_NP1 published_VVN in_II 2008_MC ._. 
In_RR21 addition_RR22 ,_, other_JJ peer-reviewed_JJ scientific_JJ assessments_NN2 were_VBDR used_VVN ,_, including_II those_DD2 of_IO the_AT Intergovernmental_JJ Panel_NN1 on_II Climate_NN1 Change_NN1 ,_, the_AT U.S._NP1 National_JJ Assessment_NN1 of_IO the_AT Consequences_NN2 of_IO Climate_NN1 Variability_NN1 and_CC Change_NN1 ,_, the_AT Arctic_JJ Climate_NN1 Impact_NN1 Assessment_NN1 ,_, the_AT National_JJ Research_NN1 Council_NN1 's_GE Transportation_NN1 Research_NN1 Board_NN1 report_NN1 on_II the_AT Potential_JJ Impacts_NN2 of_IO Climate_NN1 Change_NN1 on_II U.S._NP1 Transportation_NN1 ,_, and_CC a_AT1 variety_NN1 of_IO regional_JJ climate_NN1 impact_NN1 assessments_NN2 ._. 
These_DD2 assessments_NN2 were_VBDR augmented_VVN with_IW government_NN1 statistics_NN as_CSA necessary_JJ (_( such_II21 as_II22 population_NN1 census_NN1 and_CC energy_NN1 usage_NN1 )_) as_II31 well_II32 as_II33 publicly_RR available_JJ observations_NN2 and_CC peer-reviewed_JJ research_NN1 published_VVN through_II the_AT end_NN1 of_IO 2008_MC ._. 
This_DD1 new_JJ work_NN1 was_VBDZ carefully_RR selected_VVN by_II the_AT author_NN1 team_NN1 with_IW advice_NN1 from_II expert_NN1 reviewers_NN2 to_TO update_VVI key_JJ aspects_NN2 of_IO climate_NN1 change_NN1 science_NN1 relevant_JJ to_II this_DD1 report_NN1 ._. 
The_AT icons_NN2 on_II the_AT bottom_NN1 of_IO this_DD1 page_NN1 represent_VV0 some_DD of_IO the_AT major_JJ sources_NN2 drawn_VVN upon_II for_IF this_DD1 synthesis_NN1 report_NN1 ._. 
On_II the_AT first_MD page_NN1 of_IO each_DD1 major_JJ section_NN1 ,_, the_AT sources_NN2 primarily_RR drawn_VVN upon_II for_IF that_DD1 section_NN1 are_VBR shown_VVN using_VVG these_DD2 icons_NN2 ._. 
Endnotes_NN2 ,_, indicated_VVN by_II superscript_NN1 numbers_NN2 and_CC compiled_VVN at_II the_AT end_NN1 of_IO the_AT book_NN1 ,_, are_VBR used_VVN for_IF specific_JJ references_NN2 throughout_II the_AT report_NN1 ._. 
This_DD1 introduction_NN1 to_II global_JJ climate_NN1 change_NN1 explains_VVZ very_RG briefly_RR what_DDQ has_VHZ been_VBN happening_VVG to_II the_AT world_NN1 's_GE climate_NN1 and_CC why_RRQ ,_, and_CC what_DDQ is_VBZ projected_VVN to_TO happen_VVI in_II the_AT future_NN1 ._. 
While_CS this_DD1 report_NN1 focuses_VVZ on_II climate_NN1 change_NN1 impacts_NN2 in_II the_AT United_NP1 States_NP1 ,_, understanding_VVG these_DD2 changes_NN2 and_CC their_APPGE impacts_NN2 requires_VVZ an_AT1 understanding_NN1 of_IO the_AT global_JJ climate_NN1 system_NN1 ._. 
Many_DA2 changes_NN2 have_VH0 been_VBN observed_VVN in_II global_JJ climate_NN1 over_II the_AT past_JJ century_NNT1 ._. 
The_AT nature_NN1 and_CC causes_NN2 of_IO these_DD2 changes_NN2 have_VH0 been_VBN comprehensively_RR chronicled_VVN in_II a_AT1 variety_NN1 of_IO recent_JJ reports_NN2 ,_, such_II21 as_II22 those_DD2 by_II the_AT Intergovernmental_JJ Panel_NN1 on_II Climate_NN1 Change_NN1 (_( IPCC_NP1 )_) and_CC the_AT U.S._NP1 Climate_NN1 Change_NN1 Science_NN1 Program_NN1 (_( CCSP_NP1 )_) ._. 
This_DD1 section_NN1 does_VDZ not_XX intend_VVI to_TO duplicate_VVI these_DD2 comprehensive_JJ efforts_NN2 ,_, but_CCB rather_RR to_TO provide_VVI a_AT1 brief_JJ synthesis_NN1 ,_, and_CC to_TO integrate_VVI more_RGR recent_JJ work_NN1 with_IW the_AT assessments_NN2 of_IO the_AT IPCC_NP1 ,_, CCSP_NP1 and_CC others_NN2 ._. 
Human_JJ activities_NN2 have_VH0 led_VVN to_II large_JJ increases_NN2 in_II heat-trapping_JJ gases_NN2 over_II the_AT past_JJ century_NNT1 ._. 
The_AT Earth_NN1 's_GE climate_NN1 depends_VVZ on_II the_AT functioning_NN1 of_IO a_AT1 natural_JJ "_" greenhouse_NN1 effect_NN1 ._. "_" 
This_DD1 effect_NN1 is_VBZ the_AT result_NN1 of_IO heat-trapping_JJ gases_NN2 (_( also_RR known_VVN as_II greenhouse_NN1 gases_NN2 )_) like_II water_NN1 vapor_NN1 ,_, carbon_NN1 dioxide_NN1 ,_, ozone_NN1 ,_, methane_NN1 ,_, and_CC nitrous_JJ oxide_NN1 ,_, which_DDQ absorb_VV0 heat_NN1 radiated_VVD from_II the_AT Earth_NN1 's_GE surface_NN1 and_CC lower_JJR atmosphere_NN1 and_CC then_RT radiate_VV0 much_DA1 of_IO the_AT energy_NN1 back_RP toward_II the_AT surface_NN1 ._. 
Without_IW this_DD1 natural_JJ greenhouse_NN1 effect_NN1 ,_, the_AT average_JJ surface_NN1 temperature_NN1 of_IO the_AT Earth_NN1 would_VM be_VBI about_II 60&deg;F_FO colder_JJR ._. 
However_RR ,_, human_JJ activities_NN2 have_VH0 been_VBN releasing_VVG additional_JJ heat-trapping_JJ gases_NN2 ,_, intensifying_VVG the_AT natural_JJ greenhouse_NN1 effect_NN1 ,_, thereby_RR changing_VVG the_AT Earth_NN1 's_GE climate_NN1 ._. 
Climate_NN1 is_VBZ influenced_VVN by_II a_AT1 variety_NN1 of_IO factors_NN2 ,_, both_RR human-induced_JJ and_CC natural_JJ ._. 
The_AT increase_NN1 in_II the_AT carbon_NN1 dioxide_NN1 concentration_NN1 has_VHZ been_VBN the_AT principal_JJ factor_NN1 causing_VVG warming_VVG over_II the_AT past_JJ 50_MC years_NNT2 ._. 
Its_APPGE concentration_NN1 has_VHZ been_VBN building_VVG up_RP in_II the_AT Earth_NN1 's_GE atmosphere_NN1 since_CS the_AT beginning_NN1 of_IO the_AT industrial_JJ era_NN1 in_II the_AT mid-1700s_MC2 ,_, primarily_RR due_II21 to_II22 the_AT burning_JJ of_IO fossil_NN1 fuels_NN2 (_( coal_NN1 ,_, oil_NN1 ,_, and_CC natural_JJ gas_NN1 )_) and_CC the_AT clearing_NN1 of_IO forests_NN2 ._. 
Human_JJ activities_NN2 have_VH0 also_RR increased_VVN the_AT emissions_NN2 of_IO other_JJ greenhouse_NN1 gases_NN2 ,_, such_II21 as_II22 methane_NN1 ,_, nitrous_JJ oxide_NN1 ,_, and_CC halocarbons_NN2 ._. 
These_DD2 emissions_NN2 are_VBR thickening_VVG the_AT blanket_NN1 of_IO heat-trapping_JJ gases_NN2 in_II Earth_NN1 's_GE atmosphere_NN1 ,_, causing_VVG surface_NN1 temperatures_NN2 to_TO rise_VVI ._. 
Heat-trapping_JJ gases_NN2 Carbon_NN1 dioxide_NN1 concentration_NN1 has_VHZ increased_VVN due_II21 to_II22 the_AT use_NN1 of_IO fossil_NN1 fuels_NN2 in_II electricity_NN1 generation_NN1 ,_, transportation_NN1 ,_, and_CC industrial_JJ and_CC household_NN1 uses_NN2 ._. 
It_PPH1 is_VBZ also_RR produced_VVN as_II a_AT1 by-product_NN1 during_II the_AT manufacturing_NN1 of_IO cement_NN1 ._. 
Deforestation_NN1 provides_VVZ a_AT1 source_NN1 of_IO carbon_NN1 dioxide_NN1 and_CC reduces_VVZ its_APPGE uptake_NN1 by_II trees_NN2 and_CC other_JJ plants_NN2 ._. 
Globally_RR ,_, over_II the_AT past_JJ several_DA2 decades_NNT2 ,_, about_RG 80_MC percent_NNU of_IO human-induced_JJ carbon_NN1 dioxide_NN1 emissions_NN2 came_VVD from_II the_AT burning_JJ of_IO fossil_NN1 fuels_NN2 ,_, while_CS about_RG 20_MC percent_NNU resulted_VVN from_II deforestation_NN1 and_CC associated_JJ agricultural_JJ practices_NN2 ._. 
The_AT concentration_NN1 of_IO carbon_NN1 dioxide_NN1 in_II the_AT atmosphere_NN1 has_VHZ increased_VVN by_II roughly_RR 35_MC percent_NNU since_CS the_AT start_NN1 of_IO the_AT industrial_JJ revolution_NN1 ._. 
Methane_NN1 concentration_NN1 has_VHZ increased_VVN mainly_RR as_II a_AT1 result_NN1 of_IO agriculture_NN1 ;_; raising_VVG livestock_NN (_( which_DDQ produce_VV0 methane_NN1 in_II their_APPGE digestive_JJ tracts_NN2 )_) ;_; mining_NN1 ,_, transportation_NN1 ,_, and_CC use_NN1 of_IO certain_JJ fossil_NN1 fuels_NN2 ;_; sewage_NN1 ;_; and_CC decomposing_JJ garbage_NN1 in_II landfills_NN2 ._. 
About_RG 70_MC percent_NNU of_IO the_AT emissions_NN2 of_IO atmospheric_JJ methane_NN1 are_VBR now_RT related_VVN to_II human_JJ activities_NN2 ._. 
Nitrous_JJ oxide_NN1 concentration_NN1 is_VBZ increasing_VVG as_II a_AT1 result_NN1 of_IO fertilizer_NN1 use_NN1 and_CC fossil_NN1 fuel_NN1 burning_VVG ._. 
Halocarbon_NN1 emissions_NN2 come_VV0 from_II the_AT release_NN1 of_IO certain_JJ manufactured_JJ chemicals_NN2 to_II the_AT atmosphere_NN1 ._. 
Examples_NN2 include_VV0 chlorofluorocarbons_NN2 (_( CFCs_NN2 )_) ,_, which_DDQ were_VBDR used_VVN extensively_RR in_II refrigeration_NN1 and_CC for_IF other_JJ industrial_JJ processes_NN2 before_II their_APPGE presence_NN1 in_II the_AT atmosphere_NN1 was_VBDZ found_VVN to_TO cause_VVI stratospheric_JJ ozone_NN1 depletion_NN1 ._. 
The_AT abundance_NN1 of_IO these_DD2 gases_NN2 in_II the_AT atmosphere_NN1 is_VBZ now_RT decreasing_VVG as_II a_AT1 result_NN1 of_IO international_JJ regulations_NN2 designed_VVN to_TO protect_VVI the_AT ozone_NN1 layer_NN1 ._. 
Continued_JJ decreases_NN2 in_II ozone-depleting_JJ halocarbon_NN1 emissions_NN2 are_VBR expected_VVN to_TO reduce_VVI their_APPGE relative_JJ influence_NN1 on_II climate_NN1 change_NN1 in_II the_AT future_NN1 ._. 
Many_DA2 halocarbon_NN1 replacements_NN2 ,_, however_RR ,_, are_VBR potent_JJ greenhouse_NN1 gases_NN2 ,_, and_CC their_APPGE concentrations_NN2 are_VBR increasing_VVG ._. 
Ozone_NN1 is_VBZ a_AT1 greenhouse_NN1 gas_NN1 ,_, and_CC is_VBZ continually_RR produced_VVN and_CC destroyed_VVN in_II the_AT atmosphere_NN1 by_II chemical_JJ reactions_NN2 ._. 
In_II the_AT troposphere_NN1 ,_, the_AT lowest_JJT 5_MC to_II 10_MC miles_NNU2 of_IO the_AT atmosphere_NN1 near_II the_AT surface_NN1 ,_, human_JJ activities_NN2 have_VH0 increased_VVN the_AT ozone_NN1 concentration_NN1 through_II the_AT release_NN1 of_IO gases_NN2 such_II21 as_II22 carbon_NN1 monoxide_NN1 ,_, hydrocarbons_NN2 ,_, and_CC nitrogen_NN1 oxides_NN2 ._. 
These_DD2 gases_NN2 undergo_VV0 chemical_JJ reactions_NN2 to_TO produce_VVI ozone_NN1 in_II the_AT presence_NN1 of_IO sunlight_NN1 ._. 
In_II31 addition_II32 to_II33 trapping_VVG heat_NN1 ,_, excess_JJ ozone_NN1 in_II the_AT troposphere_NN1 causes_VVZ respiratory_JJ illnesses_NN2 and_CC other_JJ human_JJ health_NN1 problems_NN2 ._. 
In_II the_AT stratosphere_NN1 ,_, the_AT layer_NN1 above_II the_AT troposphere_NN1 ,_, ozone_NN1 exists_VVZ naturally_RR and_CC protects_VVZ life_NN1 on_II Earth_NN1 from_II exposure_NN1 to_II excessive_JJ ultraviolet_JJ radiation_NN1 from_II the_AT Sun_NN1 ._. 
As_CSA mentioned_VVN previously_RR ,_, halocarbons_NN2 released_VVN by_II human_JJ activities_NN2 destroy_VV0 ozone_NN1 in_II the_AT stratosphere_NN1 and_CC have_VH0 caused_VVN the_AT ozone_NN1 hole_NN1 over_II Antarctica_NP1 ._. 
Changes_NN2 in_II the_AT stratospheric_JJ ozone_NN1 layer_NN1 have_VH0 contributed_VVN to_II changes_NN2 in_II wind_NN1 patterns_NN2 and_CC regional_JJ climates_NN2 in_II Antarctica_NP1 ._. 
Water_NN1 vapor_NN1 is_VBZ the_AT most_RGT important_JJ and_CC abundant_JJ greenhouse_NN1 gas_NN1 in_II the_AT atmosphere_NN1 ._. 
Human_JJ activities_NN2 produce_VV0 only_RR a_AT1 very_RG small_JJ increase_NN1 in_II water_NN1 vapor_NN1 through_II irrigation_NN1 and_CC combustion_NN1 processes_NN2 ._. 
However_RR ,_, the_AT surface_NN1 warming_VVG caused_VVN by_II human-produced_JJ increases_NN2 in_II other_JJ greenhouse_NN1 gases_NN2 leads_VVZ to_II an_AT1 increase_NN1 in_II atmospheric_JJ water_NN1 vapor_NN1 ,_, since_CS a_AT1 warmer_JJR climate_NN1 increases_VVZ evaporation_NN1 and_CC allows_VVZ the_AT atmosphere_NN1 to_TO hold_VVI more_DAR moisture_NN1 ._. 
This_DD1 creates_VVZ an_AT1 amplifying_JJ "_" feedback_NN1 loop_NN1 ,_, "_" leading_VVG to_II more_RGR warming_JJ ._. 
Other_JJ human_JJ influences_NN2 In_II31 addition_II32 to_II33 the_AT global-scale_JJ climate_NN1 effects_NN2 of_IO heat-trapping_JJ gases_NN2 ,_, human_JJ activities_NN2 also_RR produce_VV0 additional_JJ local_JJ and_CC regional_JJ effects_NN2 ._. 
Some_DD of_IO these_DD2 activities_NN2 partially_RR offset_VV0 the_AT warming_NN1 caused_VVN by_II greenhouse_NN1 gases_NN2 ,_, while_CS others_NN2 increase_VV0 the_AT warming_NN1 ._. 
One_MC1 such_DA influence_NN1 on_II climate_NN1 is_VBZ caused_VVN by_II tiny_JJ particles_NN2 called_VVN "_" aerosols_NN2 "_" (_( not_XX to_TO be_VBI confused_VVN with_IW aerosol_NN1 spray_NN1 cans_NN2 )_) ._. 
For_REX21 example_REX22 ,_, the_AT burning_JJ of_IO coal_NN1 produces_VVZ emissions_NN2 of_IO sulfur-containing_JJ compounds_NN2 ._. 
These_DD2 compounds_NN2 form_VV0 "_" sulfate_NN1 aerosol_NN1 "_" particles_NN2 ,_, which_DDQ reflect_VV0 some_DD of_IO the_AT incoming_JJ sunlight_NN1 away_II21 from_II22 the_AT Earth_NN1 ,_, causing_VVG a_AT1 cooling_JJ influence_NN1 at_II the_AT surface_NN1 ._. 
Sulfate_NN1 aerosols_NN2 also_RR tend_VV0 to_TO make_VVI clouds_NN2 more_RGR efficient_JJ at_II reflecting_VVG sunlight_NN1 ,_, causing_VVG an_AT1 additional_JJ indirect_JJ cooling_JJ effect_NN1 ._. 
Another_DD1 type_NN1 of_IO aerosol_NN1 ,_, often_RR referred_VVN to_II as_CSA soot_NN1 or_CC black_JJ carbon_NN1 ,_, absorbs_VVZ incoming_JJ sunlight_NN1 and_CC traps_NN2 heat_VV0 in_II the_AT atmosphere_NN1 ._. 
Thus_RR ,_, depending_II21 on_II22 their_APPGE type_NN1 ,_, aerosols_NN2 can_VM either_RR mask_VVI or_CC increase_VVI the_AT warming_NN1 caused_VVN by_II increased_JJ levels_NN2 of_IO greenhouse_NN1 gases_NN2 ._. 
On_II a_AT1 globally_RR averaged_JJ basis_NN1 ,_, the_AT sum_NN1 of_IO these_DD2 aerosol_NN1 effects_NN2 offsets_VVZ some_DD of_IO the_AT warming_NN1 caused_VVN by_II heat-trapping_JJ gases_NN2 ._. 
The_AT effects_NN2 of_IO various_JJ greenhouse_NN1 gases_NN2 and_CC aerosol_NN1 particles_NN2 on_II Earth_NN1 's_GE climate_NN1 depend_VV0 in_RR21 part_RR22 on_II how_RGQ long_RR these_DD2 gases_NN2 and_CC particles_NN2 remain_VV0 in_II the_AT atmosphere_NN1 ._. 
After_CS emission_NN1 ,_, the_AT atmospheric_JJ concentration_NN1 of_IO carbon_NN1 dioxide_NN1 remains_VVZ elevated_JJ for_IF thousands_NNO2 of_IO years_NNT2 ,_, and_CC that_DD1 of_IO methane_NN1 for_IF decades_NNT2 ,_, while_CS the_AT elevated_JJ concentrations_NN2 of_IO aerosols_NN2 only_RR persist_VV0 for_IF days_NNT2 to_II weeks_NNT2 ._. 
The_AT climate_NN1 effects_NN2 of_IO reductions_NN2 in_II emissions_NN2 of_IO carbon_NN1 dioxide_NN1 and_CC other_JJ long-lived_JJ gases_NN2 do_VD0 not_XX become_VVI apparent_JJ for_IF at_RR21 least_RR22 several_DA2 decades_NNT2 ._. 
In_II contrast_NN1 ,_, reductions_NN2 in_II emissions_NN2 of_IO short-lived_JJ compounds_NN2 can_VM have_VHI a_AT1 rapid_JJ ,_, but_CCB complex_JJ effect_NN1 since_CS the_AT geographic_JJ patterns_NN2 of_IO their_APPGE climatic_JJ influence_NN1 and_CC the_AT resulting_JJ surface_NN1 temperature_NN1 responses_NN2 are_VBR quite_RG different_JJ ._. 
One_MC1 modeling_NN1 study_NN1 found_VVD that_CST while_CS the_AT greatest_JJT emissions_NN2 of_IO short-lived_JJ pollutants_NN2 in_II summertime_NNT1 by_II late_RR this_DD1 century_NNT1 are_VBR projected_VVN to_TO come_VVI from_II Asia_NP1 ,_, the_AT strongest_JJT climate_NN1 response_NN1 is_VBZ projected_VVN to_TO be_VBI over_II the_AT central_JJ United_NP1 States_NP1 ._. 
Human_JJ activities_NN2 have_VH0 also_RR changed_VVN the_AT land_NN1 surface_NN1 in_II ways_NN2 that_CST alter_VV0 how_RGQ much_DA1 heat_NN1 is_VBZ reflected_VVN or_CC absorbed_VVN by_II the_AT surface_NN1 ._. 
Such_DA changes_NN2 include_VV0 the_AT cutting_NN1 and_CC burning_JJ of_IO forests_NN2 ,_, the_AT replacement_NN1 of_IO other_JJ areas_NN2 of_IO natural_JJ vegetation_NN1 with_IW agriculture_NN1 and_CC cities_NN2 ,_, and_CC large-scale_JJ irrigation_NN1 ._. 
These_DD2 transformations_NN2 of_IO the_AT land_NN1 surface_NN1 can_VM cause_VVI local_JJ (_( and_CC even_RR regional_JJ )_) warming_VVG or_CC cooling_VVG ._. 
Globally_RR ,_, the_AT net_JJ effect_NN1 of_IO these_DD2 changes_NN2 has_VHZ probably_RR been_VBN a_AT1 slight_JJ cooling_NN1 of_IO the_AT Earth_NN1 's_GE surface_NN1 over_II the_AT past_JJ 100_MC years_NNT2 ._. 
Natural_JJ influences_NN2 Two_MC important_JJ natural_JJ factors_NN2 also_RR influence_VV0 climate_NN1 :_: the_AT Sun_NN1 and_CC volcanic_JJ eruptions_NN2 ._. 
Over_II the_AT past_JJ three_MC decades_NNT2 ,_, human_JJ influences_NN2 on_II climate_NN1 have_VH0 become_VVN increasingly_RR obvious_JJ ,_, and_CC global_JJ temperatures_NN2 have_VH0 risen_VVN sharply_RR ._. 
During_II the_AT same_DA period_NN1 ,_, the_AT Sun_NN1 's_GE energy_NN1 output_NN1 (_( as_CSA measured_VVN by_II satellites_NN2 since_II 1979_MC )_) has_VHZ followed_VVN its_APPGE historical_JJ 11-year_JJ cycle_NN1 of_IO small_JJ ups_NN2 and_CC downs_NN2 ,_, but_CCB with_IW no_AT net_JJ increase_NN1 (_( see_VV0 figure_NN1 page_NN1 20_MC )_) ._. 
The_AT two_MC major_JJ volcanic_JJ eruptions_NN2 of_IO the_AT past_JJ 30_MC years_NNT2 have_VH0 had_VHN short-term_JJ cooling_JJ effects_NN2 on_II climate_NN1 ,_, lasting_VVG 2_MC to_II 3_MC years_NNT2 ._. 
Thus_RR ,_, these_DD2 natural_JJ factors_NN2 can_VM not_XX explain_VVI the_AT warming_NN1 of_IO recent_JJ decades_NNT2 ;_; in_II fact_NN1 ,_, their_APPGE net_JJ effect_NN1 on_II climate_NN1 has_VHZ probably_RR been_VBN a_AT1 slight_JJ cooling_JJ influence_NN1 over_II this_DD1 period_NN1 ._. 
Slow_JJ changes_NN2 in_II Earth_NN1 's_GE orbit_NN1 around_II the_AT Sun_NN1 and_CC its_APPGE tilt_NN1 toward_II or_CC away_II21 from_II22 the_AT Sun_NN1 are_VBR also_RR a_AT1 purely_RR natural_JJ influence_NN1 on_II climate_NN1 ,_, but_CCB are_VBR only_RR important_JJ on_II timescales_NN2 from_II thousands_NNO2 to_II many_DA2 tens_MC2 of_IO thousands_NNO2 of_IO years_NNT2 ._. 
The_AT climate_NN1 changes_NN2 that_CST have_VH0 occurred_VVN over_II the_AT last_MD century_NNT1 are_VBR not_XX solely_RR caused_VVN by_II the_AT human_NN1 and_CC natural_JJ factors_NN2 described_VVN above_RL ._. 
In_II31 addition_II32 to_II33 these_DD2 influences_NN2 ,_, there_EX are_VBR also_RR fluctuations_NN2 in_II climate_NN1 that_CST occur_VV0 even_RR in_II the_AT absence_NN1 of_IO changes_NN2 in_II human_JJ activities_NN2 ,_, the_AT Sun_NN1 ,_, or_CC volcanoes_NN2 ._. 
One_MC1 example_NN1 is_VBZ the_AT El_NP1 Ni_NP1 ?_? o_ZZ1 phenomenon_NN1 ,_, which_DDQ has_VHZ important_JJ influences_NN2 on_II many_DA2 aspects_NN2 of_IO regional_JJ and_CC global_JJ climate_NN1 ._. 
Many_DA2 other_JJ modes_NN2 of_IO variability_NN1 have_VH0 been_VBN identified_VVN by_II climate_NN1 scientists_NN2 and_CC their_APPGE effects_NN2 on_II climate_NN1 occur_VV0 at_II the_AT same_DA time_NNT1 as_CSA the_AT effects_NN2 of_IO human_JJ activities_NN2 ,_, the_AT Sun_NN1 ,_, and_CC volcanoes_NN2 ._. 
Carbon_NN1 release_NN1 and_CC uptake_NN1 Once_RR carbon_NN1 dioxide_NN1 is_VBZ emitted_VVN to_II the_AT atmosphere_NN1 ,_, some_DD of_IO it_PPH1 is_VBZ absorbed_VVN by_II the_AT oceans_NN2 and_CC taken_VVN up_RP by_II vegetation_NN1 ,_, although_CS this_DD1 storage_NN1 may_VM be_VBI temporary_JJ ._. 
About_RG 45_MC percent_NNU of_IO the_AT carbon_NN1 dioxide_NN1 emitted_VVD by_II human_JJ activities_NN2 in_II the_AT last_MD 50_MC years_NNT2 is_VBZ now_RT stored_VVN in_II the_AT oceans_NN2 and_CC vegetation_NN1 ._. 
