1 6 , 0 0 0 ▁of ▁Europe ' s ▁usual ▁ 2 8 , 0 0 0 ▁daily ▁scheduled ▁passenger ▁fl ights ▁were ▁cancel led ▁and ▁on ▁the ▁following ▁day ▁ 1 6 , 0 0 0 ▁of ▁the ▁usual ▁ 2 2 , 0 0 0 ▁fl ights ▁were ▁cancel led . ▁By ▁ 2 1 ▁April ▁ 9 5 , 0 0 0 ▁fl ights ▁had ▁been ▁cancel led . ▁ ▁Before ▁this , ▁the ▁sever est ▁restrictions ▁to ▁air ▁travel ▁in ▁recent ▁times ▁were ▁following ▁the ▁attacks ▁of ▁ 1 1 ▁September ▁ 2 0 0 1 ▁in ▁the ▁United ▁States ▁when ▁all ▁civil ▁air ▁traffic ▁( not ▁just ▁scheduled ) ▁in ▁US ▁air space , ▁and ▁to ▁and ▁from ▁the ▁United ▁States , ▁was ▁ground ed ▁for ▁three ▁days . ▁ ▁Professor ▁Bill ▁Mc Gu ire ▁of ▁the ▁A on ▁Ben field ▁U CL ▁Haz ard ▁Research ▁Centre ▁said ▁on ▁ 1 5 ▁April ▁that ▁the ▁most ▁notable ▁er u ption ▁of ▁the ▁I cel and ic ▁vol cano ▁L aki ▁which ▁occurred ▁in ▁ 1 7 8 3 , ▁would , ▁if ▁it ▁occurred ▁today ▁" have ▁the ▁potential ▁to ▁sever ely ▁affect ▁air ▁travel ▁at ▁high ▁northern ▁lat itudes ▁for ▁six ▁months ▁or ▁more ". ▁The ▁last ▁time ▁the ▁E y ja f j alla jö k ull ▁vol cano ▁er upt ed , ▁in ▁ 1 8 2 1 , ▁it ▁spe wed ▁ash ▁for ▁over ▁a ▁year . ▁Ge oph ys ic ists ▁in