Lecture 12b Recent Warming

The Main Evidence The Global Temperature Record: today

The Global Historical Climate Network (GHCN)

Global Mean Temperature – Instrumental Period Source: / / Temperature Anomaly ( o C) Note the strong positive anomalies 1998, 2010

Instrumental Record – Global Mean Temperature Global mean temperature has risen approximately 0.6 to 1.0 °C in last 100 years Most of the increase occurred in two stages: and 1975 – today Global temperature changed little in period Global temperature changed little in period

Causes of 20 th -Century Warming Recall: Large increases in greenhouse gas concentrations, esp. CO 2 Recall: Large increases in greenhouse gas concentrations, esp. CO 2 Seems likely that this is main cause Seems likely that this is main cause

A Thorny Question Surface temperature record is much “noisier” than CO 2 record. Why? Answer: Surface temp. has more influences than atmospheric CO 2.

Some Natural Factors Influencing Surface Temp Natural variability Example: El Niño/La Niña cycle Example: El Niño/La Niña cycle External forcing Solar flux variations (more later) Solar flux variations (more later) Particles from volcanic eruptions Particles from volcanic eruptions

“Normal” Temperatures in Equatorial Pacific “Anomaly” is the deviation from the average.

El Niño Temperatures

El Niño In El Niño years, surface temperature is slightly higher than normal

Global Mean Temperature – Instrumental Period Source: / / 1998 El Niño

Another sources of “noise” Volcanic eruptions (see next slide)

Troposphere Stratosphere Incoming Sunlight Volcano Formation of Particles Transport by Winds Reflection Photochemistry Another Source of Noise: Volcanoes

Volcanoes Effect on Global Temperature A large volcanic eruption can cool the Earth for a few years Example: Mt. Pinatubo eruption in Philippines in summer of 1991

Global Mean Temperature – Instrumental Period Source: / / Mt. Pintubo eruption

Bottom Line Fluctuations are to be expected due to natural processes. However, recent rising trend can only be explained by increased greenhouse effect

Another Interesting Question Was the rate of warming during the past century unusual? To answer, need proxy data. Main types: tree rings, ice cores ming/proxydata.html ming/proxydata.html ming/proxydata.html ming/proxydata.html

Temperatures of Past Millennium

Answer to Question The warming trend of the past 100 years appears to be unusually large when compared to previous 9 centuries