ggplot2: Decadal Trend Rates in Global Temperature

Charts & Graphs blog has posted GISS Anomaly Trend Chart with Decadal Trend Rates created using base graphics in R and has invited me to prepare a ggplot2 version of the same.

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Data Preparation

• The data import process is described in a more detail in a previous post.
• Convert years and months into yearmon class.
• Calculate decades from annual data and coefficients by decade.

> library(ggplot2)

> library(zoo) > dfm$yearmo <- with(dfm, paste(Month, Year)) > dfm$yearmo <- as.yearmon(dfm$yearmo, "%b %Y") > dfm <- dfm[order(dfm$yearmo), ] > dfm$decade <- dfm$Year%/%10 * 10 > dfm <- ddply(dfm, .(decade), transform, a = coef(lm(value/100 ~ + yearmo))[1])

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Plotting

First setup the graph

> p <- ggplot(dfm, aes(yearmo, value/100)) + opts(legend.position = "none", + title = "GISS Temperature Anomaly with Trend Rates By Decade") + + xlab("") + ylab("GISS Temperature Anomaly - C") + + theme_bw()

Plot annual temperature anomaly

> p + geom_line(colour = "grey80")

Add overall trend

> last_plot() + geom_smooth(method = lm, se = F, colour = "green")

Add trend by decade

> last_plot() + geom_smooth(aes(group = factor(decade), + colour = (a > 0)), method = lm, se = F)

Fix colour mapping

> last_plot() + scale_colour_manual(values = c("blue", + "red"))

Remove Legend

> last_plot() + opts(legend.position = "none")

Add text labels

> trend_rate <- ddply(dfm, .(decade), summarise, b = coef(lm(value/100 ~ + yearmo))[2] * 100) > last_plot() + geom_text(data = trend_rate, aes(decade + + 5, -0.9, label = round(b, 1), colour = (b <= 0)), + size = 4)

Annotate the graph

> b_flm <- with(dfm, round(coef(lm(value ~ yearmo)), 3))[2]

> last_plot() + annotate(x = 1880, y = 0.7, geom = "text", + hjust = 0, label = paste("Overall Trend - oC/C \n", + b_flm), size = 4, colour = "darkgreen") + annotate(x = 1920, + y = -0.8, geom = "text", hjust = 0, label = "Decade Trend Rate -oC per Century", + size = 4, colour = "darkgreen")