WordNet (r) 3.0 (2006):

worsening
    adj 1: changing for the worse; "worried by the worsening storm"
           [ant: bettering]
    n 1: process of changing to an inferior state [syn:
         deterioration, decline in quality, declension,
         worsening]
    2: changing something with the result that it becomes worse

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "worsening":
   about-face, accommodation, adaptation, adjustment, aggravation,
   alteration, amelioration, amplification, annoyance, apostasy,
   augmentation, betterment, break, change, change of heart,
   changeableness, coming apart, constructive change, contentiousness,
   continuity, conversion, cracking, crumbling, decadent, declining,
   deepening, defection, degenerate, degeneration,
   degenerative change, deliberate aggravation, deteriorating,
   deterioration, deviation, difference, discontinuity,
   disintegrating, divergence, diversification, diversion, diversity,
   draining, drooping, dwindling, ebbing, effete, embittering,
   embitterment, enhancement, enlargement, exacerbation, exasperation,
   fading, failing, falling, fitting, flagging, flip-flop,
   fragmenting, going to pieces, gradual change, heightening,
   improvement, increase, intensification, irritation, languishing,
   magnification, marcescent, melioration, mitigation, modification,
   modulation, overthrow, pining, provocation, qualification,
   radical change, re-creation, realignment, redesign, reform,
   reformation, regressive, remaking, renewal, reshaping,
   restructuring, retrograde, retrogressive, reversal, revival,
   revivification, revolution, sharpening, shift, shriveling, sinking,
   sliding, slipping, slumping, souring, subsiding, sudden change,
   switch, tabetic, total change, transition, turn, turnabout,
   upheaval, variation, variety, violent change, waning, wasting,
   wilting, withering