AN MD80 FLT CREW EXPERIENCED LOSS OF CABIN PRESSURE ON CLIMBOUT. THEY RAN THE CHECKLIST; DECLARED AN EMERGENCY; AND RETURNED TO THEIR DEP ARPT.

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AFTER CLBING AND NEARING THE ZZZ VOR ON THE DEP; WE WERE ASKED TO EXPEDITE OUR CLB THROUGH 13000 FT ON OUR WAY TO 15000 FT. BEING PRETTY HVY; WE SAID WE WOULD DO OUR BEST AND MAINTAINED ROUGHLY 250 KTS TO COMPLY. AT THAT TIME; WE NOTICED OUR EARS STARTING TO SLOWLY POP AND IMMEDIATELY CHKED THE PACK UNITS AND ASSOCIATED INDICATORS. THERE WAS VERY LOW FLOW AND THE PRESSURIZATION FLOW LIGHT THEN ILLUMINATED. WE ALSO NOTICED THE PRESSURIZATION DIFFERENTIAL BECOMING LOWER AND THE CABIN ALT NEEDLE VERY SLOWLY INCREASING. WE OBSERVED THE OUTFLOW VALVE INDICATOR NEAR OR FULLY CLOSED AND CHOSE TO RUN THE CABIN ALT WARNING/RAPID DEPRESSURIZATION CHKLIST TO MANUALLY CLOSE THE OUTFLOW VALVE AS WELL AS VERIFY SWITCH AND INDICATOR POS IN THE PRESSURIZATION SYS. ALL SWITCHES WERE NORMAL WITH PACKS IN AUTO. WE THEN RAN THE PRESSURIZATION FLOW CHKLIST TO COMPLY WITH THE AMBER LIGHT INDICATION AND ENSURE AC SUPPLY SWITCHES WERE IN AUTO. WE IMMEDIATELY NOTIFIED ATC AND ASKED FOR A LOWER ALT BELOW 10000 FT AND AN EMER CLRNC BACK TO ZZZ TO LAND. THE APCH AND LNDG BACK INTO ZZZ WAS UNEVENTFUL.