The samples are too far apart. If you are looking at data and sampling every 200 years or every 6000 years, then how are you going to miss a giant spike that only lasts 100 years? The CO2 in the modern time frame has only been going up for a relatively short time. The scientists also threw out any spikes in the data as spurious, which would likely also miss the current habbenings. So if you are looking for short term wild swings in CO2 concentration in ice core data, you're gonna have to find someone to do brand-new sampling because all the old data we have is not suitable for the comparison to the modern day.