A global climate model that incorporated increasing carbon dioxide concentrations was created to simulate climate changes over the next 75 years. In the model, pacific sea surface temperatures increased more east of the date line. Precipitation patterns also shifted. These changes strongly resemble aspects of El Nino events, and actual observations in the 1980s. The resemblance to El Nino patterns complicates detection of climate changes due to an increase in carbon dioxide.

A global coupled general circulation model was used with CO₂ levels increasing 1% per year over a 75 year period. The CO₂ level approximately doubled at year 70 of the experiment. A fairly simple cloud albedo parametrization was used in the model. Although the effect of clouds is accounted for in a simple way, the results are similar to a more complex model that uses computed cloud optical properties. The other large assumption with this model is that the parametrizations chosen to model present day conditions will still hold far in the future. This is true in all models however, but it should still be kept in mind.

A control experiment was completed using present day values for CO₂. This data is then subtracted from the experiment with a doubling of CO₂ to see what the difference is. The increased CO₂ model shows an increase in global temperature as expected. What's interesting however is that are large differences between the results in the eastern pacific and the western pacific.

One of the differences is precipitation. It increased in the central equatorial Pacific, and decreased in Australasia and the eastern Indian Ocean. These precipitation patterns resemble those associated with El Nino events. Observations taken in the 1980s also resemble these general trends. Some other main results that show the difference between the east and west pacific can be seen below in table 1:

Table 1

	SST	TCLD	Albedo	Abs Solar	Adv	Q-con
	(C)	(%)	(%)	(Wm-2)	(Wm-2)	(%)
E Pacific	+3.49	-13	-4	+2.23	+7.65	+62
W Pacific	+2.21	-5	-1	-3.92	+5.28	+3

where:

the E Pacific is 5S-5N, 120W-90W
the W Pacific is 5S-5N, 140E-170E
SST is sea surface temperature
TCLD is total cloud amount
Abs Solar is absorbed solar at the surface
Adv is latent and ocean heat advection
Q-con is surface moisture convergence

Cloud albedo feedback decreases more in the east than it does in the west. The total cloud amount also decreases much more in the east than it does in the west. These two differences cause the absorbed solar energy to decrease in the west and increase in the east(+2.23 vs -3.92). Another difference is in ocean advection. Ocean advection is predicted to be higher for the eastern pacific. This is due to a reduction in eastern pacific upwelling which causes warmer surface temperatures in the eastern pacific ocean.

In the computer model, radiative changes (mostly due to changes in cloud amount and albedo feedback) plus changes in ocean feedbacks that reduce easterly winds (reducing eastern upwelling) combine to cause a greater relative warming east of the date line. The model also showed a shift in the normal pattern of precipitation. These changes are very similar to what occurs during an El Nino event. Actual observations in the 1980s also showed some of the same temperature and precipitation patterns that the model showed. There certainly isn't definitive proof, but an increase in CO₂ could cause lasting changes in the Pacific Ocean similar to what occurs during an El Nino event.

Reference

Meehl, Gerald and Warren Washington, 1996: El Nino-like climate change in a model with increased CO₂ concentrations Nature, 382, 56-60.