How reliable are air bubbles in ice core samples for determining historic levels of Carbon Dioxide in the atmosphere?

Answer 1

This question is related to the theory of anthropogenic (man-made) global warming. Global warming theory is based on physics, not on paleoclimate. Uncertainties in carbon dioxide measurements don't change the theory or the prediction that temperatures will rise, but by reducing the uncertainty, we can get a better estimate of climate sensitivity in the past - which helps inform projections for our current warming.

Prior to 1958, no one was routinely and reliably measuring atmospheric carbon dioxide (CO2), so we have no direct observational data about atmospheric CO2 before 1958. In order to have historical atmospheric CO2 data, scientists have had to come up with a variety of "proxy" data. The most direct proxy is CO2 concentrations in air found in bubbles trapped in ice thousands or even millions of years ago. The theory is that, over time, the concentrations of the various atmospheric gasses don't change. By determining the age of the ice layer, you can measure the concentration of CO2 therein and state, with confidence, that the atmosphere itself had that same concentration at the time the air bubble was trapped.

The method used for recovering CO2 concentration from ice is usually as follows:

• The ice core is cut into sections, and the outer layer trimmed.
• Each sample is finely ground in an airless extraction flask, then freeze dried.
• The air is collected in supercooled electropolished stainless steel traps.

This method collects all the air in the sample, so solubility is not an issue.

An important question for accurate dating is how far does the gas migrate. We know it doesn't diffuse widely, since we can recover a remarkable amount of detail. We do, however, need to take gravitational separation into account.

Ice cores cannot resolve very fine detail over hundreds of thousands of years. However, there are ice core records with better than decadal resolution over the modern era.

Ice core data shows remarkable agreement with direct CO2 concentration data. They clearly show ice ages and different ice cores also correlate extremely well with each other.

It is difficult to give an objective answer to a question such as "how reliable" something is. Where ice cores are concerned, we can answer differently for different aspects. The exact chronology - the age of the ice and the trapped air - can vary a little, and is often adjusted when better information is available. The pattern of historic CO2 is very certain, and very reliable. We know when CO2 was high, and we know when it was low.

There is a small amount of uncertainty associated with the CO2 concentration measurements, due to experimental uncertainty and degraded ice quality. However, it is important to remember that this uncertainty is dwarfed by the magnitude of past and current CO2 changes in the atmosphere. There is no issue with separating the signal from the noise.