Figure 1

Actual and Proxy of Monthly CO₂ Concentration (ppm) in Hawaii and Ontario for 1959–2016. Black line: the actual CO₂ data in Mauna, Hawaii (1959–2016) from National Oceanic and Atmospheric Administration. Blue line: the actual CO₂ data in Egbert, Ontario (2005–2016) from The World Data Center for Greenhouse Gases. Redline: the proxy of CO₂ data in Egbert, Ontario (1959–2004). Discontinuities are due to missing values. The actual Hawaii and Ontario CO₂ data show a similar increasing trend, but the CO₂ concentration in Ontario varies more than in Hawaii. In addition, the CO₂ level in Ontario reaches a peak between November and April and drops from May till July/August. The CO₂ concentration in Hawaii peaks in May and drops from June to October. Variation in the CO₂ concentration in Ontario reflect the reduction in photosynthesis during the winter months. Photosynthesis is less variable during the year in Hawaii. So, we regressed the monthly Ontario CO₂ concentration on the linear corresponding value for Hawaii from 2005 to 2016. We then used the estimated coefficients from the regression to calculate the proxy serving for Ontario for 1959–2004, by substituting the Hawaii observations into the regression equation. See Table S6 for the estimated results and Fig. S2 for the residual plot in the CO₂ regression model.