hubieran contrarrestado
-they/you had counteracted
Past perfect subjunctiveellos/ellas/ustedesconjugation ofcontrarrestar.

contrarrestar

Es probable que esta mayor concentración de gases de efecto invernadero, por sí sola, hubiera causado un calentamiento aún mayor, si los aerosoles volcánicos o procedentes del hombre no hubieran contrarrestado algunos de los calentamientos que, por lo contrario, hubieran tenido lugar.
It is likely that these increases in greenhouse gas concentrations alone would have caused more warming than observed, had volcanic and human-induced aerosols not offset some of the warming that would otherwise have taken place.
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scarecrow