# Presentation: 598487

## Resolution of Disparities in Tropospheric Temperature Records

- Ben Santer
- Program for Climate Model Diagnosis and Intercomparison
- Lawrence Livermore National Laboratory
- Livermore, CA (santer1@llnl.gov)
- U.S. Climate Change Science Program Planning Workshop
- Washington D.C., Dec. 4th, 2002

**Notes:**
 It’s an honor and a privilege for me to be able to address you.
 The work that I’ll be talking about is not mine alone. I’d like to acknowledge the contributions from my colleagues in the United States and Britain. Their contributions are much greater than the small font size that I’ve used here might indicate!

## Why have the surface and troposphere warmed at different rates?

- Some have suggested there is a significant discrepancy between:
**Strong surface warming of 0.15 - 0.20****o****C/decade recorded by thermometers**
**Weak warming of lower troposphere estimated by satellites and radiosondes**
- This discrepancy has been used to cast doubt on both climate model predictions and the reality of surface warming** **

## The Microwave Sounding Unit monitors temperatures over a broad atmospheric layer – not at Earth’s surface

***Weighting function of the MSU lower troposphere temperature retrieval (2LT) peaks at roughly 4 km above Earth’s surface***

**Notes:**
 Here is an outline of the structure of my talk.
 First I’ll discuss some of the key findings of a recent Intergovernmental report that investigated the nature and causes of climate change.
 Then I’ll define some terms that I’ll be using in my talk.
 Next I’ll try to give you some indication of the principal advances that have been made in our knowledge of the causes of climate change.
 Most of the talk will focus on discussion of one example of a so-called “fingerprint” study - a statistical comparison of patterns of climate change in observed data and in model predictions.
 Finally, in my concluding remarks, I’ll point out some of the key uncertainties in this type of work, and tell you why I believe that we all have a stake in advancing the science.

## There are differences in the spatial coverage of satellite- and thermometer-based temperature measurements

- Lower troposphere (satellite)

- Surface (thermometer)

**Annual-mean temperature anomalies (****o****C) w.r.t. 1979-93**

- 1979

- 1980

- 1998

## Estimating El Niño and volcano effects on tropospheric temperatures

- Satellite tropospheric temperature data

- El Niño index

- El Niño effect on temperature

- Satellite data minus El Niño effect

- Volcano effect on temperature

- After removing El Niño and volcanoes

**Trend in residuals: 0.11****°C/decade**

## Can we reconcile modeled and observed temperature trends?

***Can we reconcile modeled and observed temperature trends?***

***Time period for trend comparisons is 1979-1997***

***Positive trend difference: Surface warms by more than troposphere in OBS***

**BLUE: OBSERVED**
**RED:   MODEL**

***SIGNIFICANCE OF TREND DIFFERENCES***

*Different ways of removing volcano and El Ni**ño effects*

***Trend difference close to zero: Surface and troposphere show similar warming rates in MODEL***

## Observational uncertainties: A missing piece of the puzzle?

## Modeled tropospheric temperature trends are significantly different from one set of MSU observations...

***Significance of differences between modeled and observed Channel 2 trends (1979-1997)***

- Observations I (Christy/Spencer MSU)

- Model (ECHAM GSOP experiment)

## But modeled tropospheric temperature trends are in good agreement with a second set of MSU observations

***Significance of differences between modeled and observed Channel 2 trends (1979-1997)***

**Observations I (Christy/Spencer)**

**Model (ECHAM GSOP)**

**Observations II (Mears et al.)**

## Does other evidence support recent warming of the troposphere?

**It is the transition zone between the turbulently-mixed troposphere and more stably-stratified stratosphere**

- Tropopause

**Notes:**
 I haven’t shown you this, but the Earth is warming. We know this from detailed analyses of thermometric temperature records. 
 The best information we have indicates that the Earth’s mean near-surface temperature has increased by around 0.5oC over the last 100 years.
 Analyses of “reconstructed” temperature data from tree rings, ice cores, corals and boreholes indicate that the Earth is now warmer than at any time since at least 1400 A.D.
 I have shown you one example of a “fingerprint” study, in which model-predicted and observed patterns of temperature change have significant similarities. 
 These similarities generally increase with time, and they can’t be fully explained by the “noise” of natural climate fluctuations (assuming that model estimates of this noise are realistic).
 Here, I’ve shown you results for vertical cross-sections of temperature change. We and others have obtained rather similar results from analyses of geographical patterns of temperature change at the Earth’s surface.

## Are recent tropopause height changes similar in models and ‘observations’?

***Global-mean tropopause pressure changes. Anomalies relative to 1979-1997 averages***

## A simplified interpretation of tropopause height changes....

***A simplified interpretation of tropopause height changes....***

**Surface**

**More CO****2**

**Volcanic eruption**

**Nominal tropopause height**

**Troposphere**

**Lower stratosphere**

## Climate model simulations of changes in tropopause height

***All model results are from the Parallel Climate Model (PCM) of the National Center for Atmospheric Research***

## Is PCM useful for studying tropopause height changes?

## Without the Pinatubo and El Chichón volcanic eruptions, the troposphere would have warmed over 1979 to present
These eruptions cooled the troposphere more than the surface
Modeled and observed surface temperature trends are in good agreement
We still need to reconcile differences between modeled and observed tropospheric warming trends
Observational uncertainty may help to explain some of these remaining differences
Recent tropopause height changes are consistent with a warming troposphere 

**These eruptions cooled the troposphere more than the surface**
**Modeled and observed surface temperature trends are in good agreement**
**We still need to reconcile differences between modeled and observed tropospheric warming trends**
**Observational uncertainty may help to explain some of these remaining differences**
**Recent tropopause height changes are consistent with a warming troposphere **

***Conclusions***