Consistent simulations of multiple proxy responses to an abrupt climate change event.

Isotope, aerosol, and methane records document an abrupt cooling event across the Northern Hemisphere at 8.2 kiloyears before present (kyr), while separate geologic lines of evidence document the catastrophic drainage of the glacial Lakes Agassiz and Ojibway into the Hudson Bay at approximately the same time. This melt water pulse may have been the catalyst for a decrease in North Atlantic Deep Water formation and subsequent cooling around the Northern Hemisphere. However, lack of direct evidence for ocean cooling has lead to speculation that this abrupt event was purely local to Greenland and called into question this proposed mechanism. We simulate the response to this melt water pulse using a coupled general circulation model that explicitly tracks water isotopes and with atmosphere-only experiments that calculate changes in atmospheric aerosol deposition (specifically (10)Be and dust) and wetland methane emissions. The simulations produce a short period of significantly diminished North Atlantic Deep Water and are able to quantitatively match paleoclimate observations, including the lack of isotopic signal in the North Atlantic. This direct comparison with multiple proxy records provides compelling evidence that changes in ocean circulation played a major role in this abrupt climate change event.

Effects of methotrexate on acute rejection and cardiac allograft vasculopathy in heart transplant recipients.

BACKGROUND: In heart transplant recipients methotrexate has been shown to reverse recurrent/persistent acute rejection refractory to intensified conventional immunosuppression. This study sought to determine whether methotrexate produces a sustained decline of heart allograft rejection rates and renders rejection rates of patients with a history of recurrent/persistent rejection similar to those of heart transplant recipients without such history. METHODS: Rejection, infection, and cardiac allograft vasculopathy were compared in 35 patients treated with methotrexate (12 +/- 9 mg/week for 34 +/- 54 weeks) and 236 patients never given methotrexate. Because the mean time from transplantation to initiation of methotrexate was 9.4 months, patients treated without methotrexate were analyzed for events < or = 9.4 versus > 9.4 months after heart transplantation. RESULTS: Demographics, perioperative and maintenance immunosuppression, and postoperative follow-up time (58 +/- 32 vs 57 +/- 33 months) were similar in the two groups. Rejection rates decreased in both groups but remained significantly higher in the patients treated with methotrexate after initiation of therapy than in the patients treated without methotrexate more than 9.4 months after transplantation (0.15 +/- 0.16 vs 0.06 +/- 0.12 episodes/patient/month; p = 0.0014). Infection rates were higher in patients after methotrexate initiation than in patients treated without methotrexate more than 9.4 months after heart transplantation (0.17 +/- 0.24 vs 0.06 +/- 0.13 episodes/patient/month; p = 0.015). At the end of the follow-up period methotrexate- and non-methotrexate-treated groups did not differ in the percentage of patients with angiographically detectable cardiac allograft vasculopathy (17.1% and 21.2%, respectively) and survival (71.4% and 64.0%, respectively). CONCLUSIONS: Even after reversal of rejection by methotrexate, patients requiring methotrexate for the treatment of persistent/recurrent rejection continued to have higher rejection rates than patients not requiring methotrexate. In spite of persistently higher rejection rates, patients treated with methotrexate did not have higher rates of cardiac allograft vasculopathy. This finding raises the question whether methotrexate provides a protective influence on the development of cardiac allograft vasculopathy in this high-risk group.