i'm not sure if i'm imagining things but in the 25 years i've been caving, it seems to be more-so in recent years that we've been getting CO2 related problems in caves causing pretty much nil access to some of them during the peak of summer months when it's at its worst. I was reading something earlier than may be a load of rubbish but might be interesting reading about how the earth and seas are sucking more co2 and wondered if this might be relevant. Would be interested in any more sciency person's views on this.
http://www.theregister.co.uk/2012/08/02/co2_absorption_doubled_since_1960/
http://www.theregister.co.uk/2012/08/02/co2_absorption_doubled_since_1960/