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The Environment / Re: What is the meaning of 400 ppm (0.04%) atmospheric CO2?
« on: 04/07/2013 23:52:35 »Quote
The months of maximum negative gradient (high photosynthetic activity) are June and July -- summer months -- while those of maximum negative gradient (low photosynthetic activity) are December, January, and February, the winter months.
Never mind the gradient, why does the CO2 level start below and increase above the trend level as the weather gets warmer? When trees become dormant they don't release more CO2 than they absorbed when active - indeed the release is negligible compared with the uptake, and in a closed system plants gradually absorb nearly all the atmospheric CO2 until there isn't enough to sustain growth. And as the weather gets warmer, humans discharge less CO2. So what is putting CO2 into the atmosphere as the sun gets higher in the sky? The annual cycle has been going on with the same amplitude and phase even when the underlying trend was much less steep than it is now.
And regarding Australian temperature records
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The Stevenson screen was first introduced to Australia in the 1880s and was installed everywhere, with a few exceptions, by 1910. Prior to this date, thermometers were located in various types of shelter, as well as under verandas and even in unheated rooms indoors. Because of this lack of standardisation, many pre-1910 temperatures in Australia are not strictly comparable with those measured after that date, and therefore must be used with care in analyses of climate change within Australia.
from Statistics Australia. Rainfall is fairly easy to measure consistently, so those stats are believable, but even with standard Stevenson screens and calibrated thermometers, point records can show point trends reliably but it's extremely difficult to extract meaningful area data or even to compare screens a mile apart if the vegetation and terrain are different. Hence my considerable caution in evaluating historic temperature data even from scientifically sophisticated sources like the Aussie outback.
The art of glider flying is to spot variations in terrain. Brown fields heat up more than green ones, and the contrast produces the thermals that give us the energy to fly. The art of farming is to introduce temporary variations in terrain by rotating crops and grazing. So you might expect the very best point measurements to show a 5 year cycle even if the climate were absolutely constant!