Data Source

The data source for this analysis is the publicly available GISTEMP data. This data is maintained by NASA’s Goddard Institute for Space Studies. The global surface temperature averages are derived from NOAA meteorological stations, ERSST ocean surface temperatures, and SCAR antarctic stations. The code below can be used in R to reproduce the data for this analysis.

library(knitr); library(dplyr); library(ggplot2); library(scales)
opts_chunk$set(fig.align="center", fig.height=5, fig.width=8, fig.path="figures/")
if(!file.exists("gisdata.txt"))
    download.file("http://data.giss.nasa.gov/gistemp/tabledata_v3/GLB.Ts+dSST.txt", "gisdata.txt")

temps <- read.table("gisdata.txt", skip=8, na.strings="***", nrows=141, stringsAsFactors=FALSE)
temps <- as.data.frame(lapply(temps,as.numeric))
temps <- temps[!is.na(temps$V1),c(1,14,15)]
names(temps) <- c("year","avTempJan","avTempDec")
#convert values from relative millidegrees C to absolute degrees F
temps <- mutate_each(temps, funs(.*.018+57.2),c(avTempJan,avTempDec))
calcNetTrends <- function (temps, referenceYear) {
    row.names(temps) <- as.character(temps$year)
    temps[paste0("xYearTrendJan",referenceYear)] <- 
        temps[referenceYear,"avTempJan"] - temps$avTempJan
    temps[paste0("xYearTrendDec",referenceYear)] <- 
        temps[referenceYear,"avTempDec"] - temps$avTempDec
    temps[paste0("numYears",referenceYear)] <- 
        temps[referenceYear,"year"] - temps$year
    temps
}
#Calculate various temperature trends to compare with Cruz's stat
temps <- calcNetTrends(temps, referenceYear="2013")
temps <- calcNetTrends(temps, referenceYear="2014")

#Caluclare all possible past trendlines for the main chart
select(na.omit(temps), year, avTempDec) %>% merge(., ., by=NULL) %>% filter(year.y<year.x) %>% mutate(trend=factor(avTempDec.x - avTempDec.y <= 0, labels=c("Warming", "No Warming"))) -> segs

#Set parameters for the main chart
start_year <- 1980
yscale <- c(57, 58.5)
ascale <- c(.2,.6)

Results

First, Cruz’s claim that there is no net warming for 17 years from 1998 - 2013 will be examined.

In this table both 2013 and 2014 are considered as the comparison year as well as whether the annual temperature average is based on the calendar year (January to December) or the seasonal year (December to November). In no case is Sen. Cruz’s claim of 17 years without warming strictly accurate. However, it appears that he was referring the 15 year period from 1998-2013, which began 17 years prior to the time of his statement and does indeed indicate a lack of warming. To find this warming trend, however, he ignored the temperature data for 2014, even though it was available at the time of his statement. As the seasonal year data is slightly more favorable to Sen. Cruz’s claims than the calendar year data, it will be used throughout this report.

maps<-matrix(c("black", "tomato3","deepskyblue2", "gold", "dashed", "solid", "solid", "dotted", "Annual Temperature","Trend with Warming", "Trend without Warming", "Ted Cruz's Trend"), nrow=4, dimnames=list(c("Temp","Warming", "No Warming", "Cruz"),c("color","linetype","label")))

ggplot(data=temps[temps$year>=start_year,], aes(x=year, y=avTempDec)) + 
    geom_segment(aes(x=year.x, xend=year.y, y=avTempDec.x, yend=avTempDec.y, color=trend, alpha=(year.x-year.y), linetype=trend), data=segs[segs$year.y>=start_year,], size=1.5)  + 
    geom_line(aes(linetype="Temp", color="Temp"), size=.5) + 
    geom_segment(aes(x=1998, xend=2013, y=temps[temps$year==1998,"avTempDec"],yend=temps[temps$year==2013,"avTempDec"], color="Cruz", linetype="Cruz"), size=1) + 
    geom_point(linetype=2,size=2) + labs(title=paste0("Temperature Trendlines ",as.character(start_year),"-2014"), x="Year",y="Global Average Surface Temp. (°F)") + 
    scale_y_continuous(limits=yscale) + scale_alpha_continuous(range=ascale) + 
    scale_color_manual(name="leg", values=maps[,"color"], breaks=rownames(maps), labels=unname(maps[,"label"])) + 
    scale_linetype_manual(name="leg", values=maps[,"linetype"], breaks=rownames(maps), labels=unname(maps[,"label"])) + 
    guides(alpha=FALSE, linetype=guide_legend(title=NULL, override.aes=list(size=.8)), color=guide_legend(title=NULL)) + 
    theme(title=element_text(size=20), axis.title.y=element_text(size=14, vjust=1), axis.title.x=element_text(size=14), axis.text=element_text(size=12), legend.text=element_text(size=10), legend.position="bottom")  

Conclusion

The claim that there has been no warming for 17 years is false. The net change in average global temperature for the 15 year period from 1998 to 2013 is a decrease, but the vast majority of trends and the overall trend clearly indicate that the planet is warming.