The stock market continued its upward climb Thursday to build upon its already historic year of gains. Traders seemed encouraged by an unexpectedly large drop in claims for unemployment benefits last week. Trading volume was very low today, however, as many portfolio managers have closed out their positions for the year. The Dow Jones Industrial average rose 122.33 points, or 0.8 percent, to 16,479.88. The Standard & Poor’s 500 index rose 8.70 points, or 0.5 percent, to 1,842.02. The Nasdaq composite added 11.76 points, or 0.3 percent, to 4,167.18.