Climate Change Characteristics Of Karst Area In SW China And Its Impacts On Karst-related Carbon Sink During Recent 40 Years

Climate change is one of the challenging environmental problems that human faces in 21 th century.Atmospheric CO₂ plays a significant role in controlling global climate.Carbonate rock is the world's biggest carbon reservoir,and the rapid kinetics of carbonate dissolution results in a large amount of atmospheric CO₂ consumption.Riverine carbon fluxes of some catchments in the world have significantly changed due to contemporary climate change and human activities.As a large region with an extensive karstic area of nearly 7.5 × 105 km2,Southwest?SW?China has experienced dramatic climate changes during recent decades.Although some studies have investigated the karst-related carbon sink in some parts of this region,the importance of climate impacts has not been assessed.This research examined the impacts of recent climate change on the karst-related carbon sink in the SW China for the period 1970-2013,using a modified maximal potential dissolution?MPD?method and GIS.We first analyzed the climate change in SW China during the past 40 years,including average temperature,precipitation,evapotranspiration,net total radiation and average wind speed,and the calculated evapotranspiration?ET?.Secondly,we analyzed major determinants of carbonate dissolution at a spatial scale during past years,calculated the total karst-related carbon sink?TCS?and carbon sink fluxes?CSFs?in the SW China karst region with different types of carbonate rocks,and then compared with other methods,and analyzed the causes of CSFs variations under the changed climate conditions.The results show that:?1?During the period 1970-2013,the mean value of temperature in SW China varied from15.04?-16.33?.The mean temperature appeared a rising trend in past 40 years and the increasing rate is 0.20 ? /decade.Mean temperature catastrophe appeared in 1993,shifting form low temperature to high temperature.The mean temperature of 1990s-2010 s was the highest in the wholestudy period and its increasing rate was 0.30?/decade.In the spatial scale,the southwest of area warmed faster thanthe west area.The mean temperature increased mostin thejunction area of Yunnan and Sichuan,south area of Yunnan,north area of Sichuan and northeast area of Chongqing in the past 40 years.The precipitation in SW China varied form 963 mm/a – 1345 mm/a,and showed a remarkable decreasing trend during 1970-2013.The decreasing rate was-17 mm/decade.Precipitation catastrophe appeared in 2003,shifting form high precipitation to low precipitation.The decreasing rate was-70 mm/decade.In the spatial scale,the middle of the area appeared most decreasing trend of precipitation.The precipitation decreased mostin junction area of Yunnan and Guizhou,middle area of Sichuan,north area of Hubei and northeast area of Chongqing in the past 40 years.?2?The annual runoff depth?D?of SW China varied from 327 mm-439 mm,with an average of 386 mm during the past 40 years.Distribution of annual Dvalues demonstrated a strong spatial gradient,with higher values in the southeast?Guangxi province and Hunan province?and lower values in the western part of the study area?Sichuan province and Yunnan province?.The mean annual Din the karst regionshowed an overall decreasing trend,with an anomaly in the 1990's where the highest D occurred?444 mm/a?;the lowest D?308 mm/a?in 2010 s.The mean [HCO3-] eq value is4.29 mmol/L in the karst region of SW China,of which thehighest and lowestvalues were observed in the western and eastern parts.The [HCO3-]eq in SW China also demonstrated a strong spatial gradient.The average equilibrium concentrations for different types of carbonates changed only slightly?<5%?over the study periods despite the changing climate conditions.The mean [HCO3-] eq showed an slightly decreasing trend during the whole period.?3?The mean value of CSFs in SW China,calculated by the modified MPD method,was approximately 9.36 t C km-2a-1,and the mean TCS is 6.85 × 106 t in the past 40 years.TCS in SW China experienced a dramatic change with regional climate,and there was a trend with TCS decreasing by about 19% from 1970 s to 2010 s.The CSFs in 1990 s was the highest,at 10.4 t C km-2a-1.After 1990 s,the value of CSFs in SW China experienced a decreasing trend until it reached the lowest value,at 5.5t C km-2a-1 in 2010 s.The distribution of CSFs showed a strong spatial gradient.There were large differences in CSFs among the provinces,attributed to differences in regional climate and to carbonate lithologies.In the period of 1970-2013,the decrease of CSFs occurred mostly in Guizhou and Yunnan provinces,which experienced larger increase in air temperature?0.33? and 1.04?,respectively?and decrease in precipitation?156 mm and 106 mm,respectively?.The decreased precipitation and runoff depth account for the most decrease of CSFs in those provinces.By contrary,the [HCO3-]eq was almost unchanged during past 40 years,thus its impacts can be ignored.?4?There exists a positive feedback between CSFs and climate change.The CSFs can respond to reginal climate change rapidly.The precipitation and temperature determine the magnitude of CSFs controlling factors runoff depth and [HCO3-]eq.The spatiotemporal changes of CSFs in SW China depended mainly on precipitation or runoff variations during the past 40 years,in whichthe chemostatic behavior of [HCO3-] was found.The carbon sink produced by carbonate weathering is sensitive to runoff depth under changed climate conditions.Some research had shown the evidence for intensification of the global water cycle.In addition,soil respiration has increased because of global warming in past fewdecades,and this trend would leads [HCO3-] increase.Thus,there will bean increase inthe karst-related carbon sink duo to the significantly positive relation between the CSFs and runoff.Based on our estimation,variation of runoff in the future will be overwhelming,for about 26 times higher compared with the change of [HCO3-].This work,thus,suggests that the karst-related carbon sink could respond to future climate change quickly,and needs to be considered in the modern global carbon cycle model.